Epidemiology is simply defined as the study of the pattern of occurrence and distribution of disease in a defined human or animal population. It is the study of the relationship between the causative agent of a particular disease, host factors, environmental factors, time factors and the occurrence of disease in a defined population. Epidemiology can also be defined as the study of the cause and distribution of a disease within a defined human population. Epidemiology is that field of medical science which is concerned with the relationship of various factors and conditions which determine the frequencies and distributions of an infectious process, a disease, or a physiologic state in a human community.

It also encompasses the study of the occurrence and distribution of infectious diseases in animal populations – especially those diseases that have tremendous negative impact on human population. Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems in the affected populations. Epidemiology is a strategy for the study of factors relating to the aetiology, prevention, and control of disease; to promote health; and to efficiently allocate efforts and resources for health promotion, maintenance and medical care in human populations. Epidemiology is a branch of public health.

Public Health is the science of protecting and improving the health of communities through education, promotion of healthy lifestyles, and research for disease and injury prevention. It is an important part of medicine that is mainly geared towards disease control and prevention rather than treatment. Public health encourages environmental and/or personal sanitation, good water distribution for a community and adequate preparation and distribution of food for public consumption – as a panacea to preventing disease outbreak. Public health professionals analyze the effect on health of genetics, personal choice and the environment in order to develop programs that protect the health of people and the community.

Public health is an interdisciplinary field and professionals in many disciplines such as nursing, medicine, veterinary medicine, dentistry, biosciences and pharmacy routinely deal with public health issues. Overall, public health is concerned with protecting the health of the entire populations. These populations can be as small as a local neighborhood, or as big as an entire country. Public health professionals try to prevent problems from happening or re-occurring through implementing educational programs, developing policies, administering services, regulating health systems and some health professions, and conducting research, in contrast to clinical professionals, such as doctors and nurses, who focus primarily on treating individuals after they become sick or injured.

The public health professional prevent disease outbreak; the clinician/physician treat diseases after disease outbreak or illness; and the epidemiologists follow the trend of disease spread in order to contain it. Public health is also a field that is concerned with limiting health disparities and a large part of public health is the fight for health care equity, quality, and accessibility.


Incidence is a measure of disease that allows us to determine a person’s probability of being diagnosed with a disease during a given period of time. Incidence refers to the occurrence of new cases of disease or injury in a population over a specified period of time. It is the number of newly diagnosed cases of a disease in a population. Incidence rate is calculated as: the number of new cases of a disease divided by the number of persons at risk for the disease.

Prevalence is a measure of disease that allows us to determine a person’s likelihood of having a disease. The number of prevalent cases is the total number of cases of disease existing in a population. A prevalence rate is the total number of cases of a disease existing in a population divided by the total population. Prevalence, sometimes referred to as prevalence rate, is the proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time. Prevalence differs from incidence in that prevalence includes all cases, both new and preexisting, in the population at the specified time, whereas incidence is limited to new cases only.

Morbidity is a term used to describe ill health. Morbidity rate shows the number of infected people in a particular population. Prevalence is a measure often used to determine the level of morbidity in a population. Measures of morbidity frequency characterize the number of persons in a population who become ill (incidence) or are ill at a given time (prevalence).

Mortality is a term used to describe death. A mortality rate is the number of deaths due to a disease divided by the total population. Mortality rate shows the number of people killed by a certain ailment in a given population.

Health is defined as the condition in which an organism (a host) and all of its parts perform its vital functions normally and properly. According to the according to the World Health organization (WHO), health is defined as a state of physical, mental and social well-being, and not merely the absence of disease or infirmity.

Disease is an impairment of the normal state of an organism or of any of its components that hinders the performance of vital functions. It is a response of a host or an organism to environmental factors (e.g. malnutrition, climates), specific infections agents (viruses, fungi, parasites, bacteria) and inherent defects of the body (e.g. various genetic anomalies).


Some of the basic reasons of studying epidemiology are due to the following facts:

  1. Diseases do not occur at random. They usually follow a pattern of occurrence.
  2. The pattern of disease occurrence in a population can be predictable.
  3. The knowledge of the pattern of occurrence of disease can be used to design control measure/strategies against the disease.

Epidemiology is studied based on models, and these models are called epidemiological models. The two models used in studying epidemiology are:

  1. Agent-host-environment model. These are the disease characteristics. Epidemiologists illustrate diseases based on the causative agent, affected hosts and the environment where it occurred.
  2. Person-place-time model. These are the disease descriptors. Epidemiologists describe disease in terms of place, time and person.


The agent-host-environment talks about the causative agent of a particular infectious disease, the particular host affected and the specific environment or community where the disease is occurring or first occurred. Here, it is the agent, host, and the environment that are implicated in the emergence of a disease. In a healthy (non-disease) state, the factors of the agent, host, and the environment remain in equilibrium (i.e. they remain balanced) as shown in Figure 1. And once this state of equilibrium is achieved, there will be no cause for alarm – because disease outbreak will rarely occur.

Figure 1. Illustration of the epidemiologic triangle

Here, none of the factors (agent, host and environment) outweigh each other. Disease only emerge when one of those factors outweigh each other. For example, if the agent outweighs the host, there will be a tilt towards the agent and a disease condition will occur. A disease condition/state can also occur if the environment tilts towards the host, and in this case the host outweighs the agent.

The Epidemiologic Triangle is a model that scientists have developed for studying health problems (Figure 2). It helps us to understand infectious diseases and how they are spread in a defined human population.

Figure 2. Illustration of the epidemiologic triad.

The Triangle has three corners (called vertices):

  1. The Agent: This is the causative agent that causes the infectious disease. And it explains the “what” of the triangle.
  2. The Host. This is the organism harboring the disease. It may be a human or animal host. It explains the “who” of the triangle.
  3. The Environment. This includes those external factors that cause or allow disease transmission in a given community. It explains the “where” of the triangle.

The mission of an epidemiologist is to break at least one of the sides of the triangle, disrupting the connection between the environment, the host, and the agent, and stopping the continuation of the disease. The disease continues to spread in the community when any part of the triangle is not broken; and in such scenarios, the disease agents still maintains high morbidity and/or mortality in the affected host population – with the prevailing environmental conditions encouraging the disease proliferation and spread.


Disease agents are the causative organisms of a disease condition in a host. They are divided into two different parts, viz:

  1. Living disease agents
  2. Non-living disease agents.

The living agents of a disease in an orderly manner (i.e. from the smallest to the biggest organism) are as follows:

  1. Prions: Prisons are infectious proteins without nucleic acids. They are the smallest disease agents and they are resistant to heat, formaldehyde and UV light.

Diseases caused by prions include:

  • Mouth cow disease of animal
  • Creutz-feldt-Jacob disease in humans
  • Scrapie of sheep and goat
  • Kuru disease of man.
  1. Viroids: Viroids are small infectious agents that cause disease of plants.
  2. Satelite: Example is hepatitis D which borrows infectivity antigen from hepatitis B in order to cause disease.
  3. Viruses
  4. Chlamydia and Rickettsia
  5. Mycoplasma
  6. Bacteria
  7. Protozoa
  8. Helminthes fungi
  9. Arthropods e.g. insects like mite and ticks

The non-living agents of disease include:

These non-living agents are in two phase – the physical and the chemical non-living agents.

The physical non-living agents include:

  • Trauma as a result of fighting
  • Extreme heat (high temperature) such as burns
  • Extreme cold e.g. Liquid N2 (at – 196oC)
  • Physical rays (UV light and X-rays).

The chemical non-living disease agents include:

  • Poisons e.g. toxins from bacteria and fungi, and organophosphates and sodium azide.
  • Nutritional deficiencies e.g. Iodine deficiency which can cause goiter, selenium deficiency which can cause muscular dystrophy and vitamin E deficiency which can cause sterility.


            The agent factors which can influence disease occurrence in an environment include:

  1. Incubation period
  2. Survivability in the environment
  3. Infectivity of the agent
  4. Invasiveness
  5. Pathogenicity/virulence of agent


Incubation period is the time lag between the entry of the causative agent into the host and the manifestation of clinical signs and symptoms of the disease by the infected host. This period varies from one agent to another. Some agents have a shorter incubation period while others have a longer incubation period. Example of short incubation period is seen in food poisoning and cholera diseases – which occur immediately after the entry of the agent into the host. Manifestation of disease conditions in the invasion of a causative agent with a long incubation period is not immediate but rather it is delayed; and this usually increases its occurrence and thus the causative agent can be easily transmitted from infected host to non-infected individuals.


This is the ability of the agent to survive and persist in the environment due to its ability to form spores which can remain in the environment when environmental conditions become unfavourable. Fungi and bacteria are notable in forming spores; and fungal and bacterial spores survive for a very long time in the environment until they find a suitable host or environment where they can thrive or proliferate. But the spores of viruses can easily be killed by environmental forces. Spores of Bacillus anthrax and fungi that survive long in the environment can easily be contracted by a susceptible host and this can increase the spread of disease in the environment.


Infectivity is the ability of the agent to cause infection. Infection is the successful entry and multiplication of the agent in the host. Some diseases can lead to high infectivity while others will have a low infectivity rate. Microorganisms that form part of the body’s normal flora such as E. coli and Candida species cause infections with low infectivity rate; and such organisms usually multiply more when the immune system of the host is depressed or compromised. There is a very low disease emergence in occurrence.


Invasiveness is the ability of the agent to move from the point of entry to other parts of the host. The more invasive the agent, the more the organs of the host becomes more affected. But if the agent is less invasive, the more localized the agent become. More invasive agents cause systemic infections that spread all over the body via blood (because they can move beyond their primary portal of entry into the body to other sites or organs of the body) while those that are localized cause localized infections (because they cannot spread beyond the point of their entry into the host’s body). A typical example of a localized infection is ring worm. More invasive agents include Mycobacterium tuberculosis (the causative agent of tuberculosis) and Treponema pallidum which causes syphilis. A localized agent cannot be spread to other parts of the host.


Pathogenicity is the ability of the agent to cause disease or harm in the host. The agent does this by destroying the host cell. The causative produces toxins that destroy the host cell by blocking of the flow of fluids in the host. Virulence is only a measure of pathogenicity. It is the severity of pathogenicity. An agent that is more virulent becomes more pathogenic than the other agents that are less virulent. Virulent agents cause more damage to the host’s cells and/or organs than less-virulent agents. The invasion of a host by a more virulent agent leads to the production of clinical signs and symptoms. The spread of the infection here will be reduced because of the awareness of people of the disease. But if the agent is not pathogenic enough, the agent will be spread more easily because no clinical signs and symptoms have been seen, which can deter people from infection.


            The host factors that influence the disease occurrence in a population includes:

  • Age of the host
  • Gender of the host
  • Tissue differences
  • Breed/racial differences
  • Occupational/functional use
  • Physiological status of the host
  • Immunological/nutritional status of the host
  • Socio-cultural influences
  • Personal habits


Age factor influences the occurrence of disease in a population. Certain infections are more associated with some age group than the orders. For example, measles are more prevalent in children than in adults. The immune system is either under developed or decreased in its function at a very early age or at an old age. Diseases like arthritis, cancer and hypertension occurs more in adults than in infants or children; and this is largely attributed to the age and low state of the immune status of the former (i.e. the elderly or adults).


Gender has a role to play in the occurrence of a disease in a population. For example gonorrhea infection occurs more in men than in women. Candidiasis is more prevalent in women than in men. Also, women suffer more from goiter due to iodine deficiency than men. Trichomoniasis which is caused by T. vaginalis causes a severe problem in women than in men who can be carriers of the infection but do not show any obvious clinical sign or symptom of the disease.


Some diseases are zoonotic in nature. Zoonoses are diseases that can be transmitted to humans through a non-human host. Some diseases can only affect animals and not affect humans or can affect man and other animals but cannot affect some species of animals. For example, Rabies can affect humans, dogs and cattle’s but it cannot infect or affect chicken because the cells of chicken are resistant to it.


The term’s breed and race are used for animals and humans respectively. Some diseases affect some races more than the others. For example, the blacks who live in the tropics are more prone to malaria than the whites who live in temperate regions of the world. Haemophilia (which is the inability of blood to clot) is more common in the white than in the blacks. Also, sickle cell anaemia is more common in the blacks than in the whites. For animals, the blue tongue disease of sheep and goat affect the foreign breeds than the local breeds. The disease, African horse sickness is a viral disease that affects Argentinean horses whereas the local breed of horses is resistant to it.


Occupation also plays an important role in the influence of disease occurrence in a population. Certain jobs that people do make them to acquire certain type of infections than the others. For example, nosocomial infections are more prevalent amongst hospital personnel than non-hospital personnel’s. Lassa fever is a disease that kills doctors and other hospital personnel’s because their profession/occupation predisposes them to the diseases via contact with infected patients sample than non-hospital personnel’s. Farmers who come in contact with the soil and dust particles are more prone to acquiring fungal infections as a result of fungal spores which they encounter in their occupation. The term “Functional use” is used for animals. E.g. Dogs can suffer from tetanus when they are taken to the farm. Also, horses used for polo, suffer from Monday morning diseases.


The physiological status of people plays some roles in the influence of disease occurrence in a population than the others. Factors like pregnancy, menstruation and puberty influences the occurrence of a disease in a defined human population. Pregnancy for example, depresses the immune system of a woman as a result of stress experienced during pregnancy. Lactation is another stress condition which exposes women to infection. Menstrual flow also exposes women to infections like occupational anemia because of the out flow of blood from their body.


A dysfunction in the immune system of a host can arise following a low nutritional status of the host. Disease condition can arise when a host is under-nourished or over nourished and when this occurs, the immune system becomes compromised or depressed and this gives way for infections to establish themselves in the host.


Some cultures make their people more prone to an infection than the others. For example, cultures that give tribal marks and those that do circumcision in females expose them to varying microbial infections. Vesico-vagina fistula (VVF) is more prone in women who do circumcision than those that abhors this practice of female genital mutilation. Some cultures also give out their female children for marriage at a very early age – which makes them prone to VVF and even preterm birth. These factors play a very important role in the occurrence of disease in a population.


Some personal habits like smoking, random sexual intercourse, drinking and eating too much influences the occurrence of disease in a population. People who smoke are more prone to getting lung cancer. Drinking and eating too much can cause cardiac problems and obesity respectively.


The environmental factors which are used as models in studying epidemiology are divided into two parts:

  1. Non-living environmental factors
  2. Living environmental factors

The non-living environmental factors include:

  • High or low temperature.
  • Rainfall / humidity
  • Windfall / direction
  • Altitude

High or low temperature: High temperature can have a direct effect on a host in terms of heat stroke. This can lead to some conditions like rashes in children. In low temperature conditions, pneumonic conditions are more prevalent because of the cold climate.

Rainfall/humidity: High rainfall leads to increases in vector-borne infections like malaria, filariasis and yellow fever. This is as a result of the increases in the number of mosquitoes and ticks in the environment following the high rainfall which gives them conditions to thrive.

Windfall/direction: The wind can carry disease agents from one place to another in different directions. This can lead to the occurrence of a disease condition in a susceptible population.

Altitude: Altitude is the height above sea level. The higher you go the cooler it becomes. Altitude plays an important role in the occurrence of disease in a population. For example, tse-tse fly infections are less common in cooler places, and also there is a lower vector-borne disease infection in such environments.

The living environmental factors include:

  • Animals, and
  • Plants

Animals serve as reservoirs and vectors of disease in an environment. Plants on the other hand provide shelter and nutrition for animals and man. The type of animals in an environment usually determines the type of vectors and disease in such environment.

Person-Place-Time Model

Person: Person here is the same thing as the host factors explained in the preliminary pages.

Place: Place here is the same thing as the environmental factors explained in the preliminary pages.

Time: Time is temporal changes in disease occurrence in a population. In terms of time, disease can occur:

  • Seasonally or yearly trend,
  • Cyclic trend, and
  • Secular trend

Seasonally/yearly trend: Here, disease occur seasonally or yearly. For example, malaria has a yearly occurrence and its occurrence is very high in rainy season. This also applies to cerebrospinal meningitis which occurs yearly and is mostly seen in hot season.

Cyclic trend: Here, disease occurs 2-3 years in cycle e.g. Rabies, Lassa fever.

Secular trend: Here, disease occurs 5-10 years or more e.g. yellow fever, influenza.


            Disease occurrence in a population occurs in the following patterns:

  1. Sporadic pattern
  2. Endemic pattern
  3. Epidemic pattern
  4. Pandemic pattern.


A sporadic disease is a disease which occurs irregularly and affects only relatively few persons in a population. It is characterized by the following features:

  1. It occurs rarely
  2. It occurs at a low frequency
  3. It takes a long inter occurrence period
  4. It is not very predictable
  5. It’s duration of occurrence is short

Examples of sporadic diseases includes: Rabies, Tetanus, Ebola and Lassa fever.


Figure 3. Sporadic disease curve.


An endemic disease is a disease that is constantly present in a population but involves relatively few persons. This disease exists at all times in a population at a frequency that may be high, low, or moderate but with minor fluctuations in frequency with time. When the occurrence of the disease is low, it is termed hypoendemic. When the occurrence is high, it is termed hyperendemic and when it is moderate, it is termed mesoendemic. Examples of endemic diseases includes: typhoid fever, malaria, and tuberculosis.

Figure 4. Endemic diseases curve.


A disease is said to occur epidemically when the occurrence is higher than the endemic or expected occurrence level within a short period of time. It is a sudden increase in the occurrence of a disease above the expected level. This means that a disease can graduate from been endemic to being epidemic. If a disease occurs for the 1st time in a population, that outbreak could amount to an epidemic because it occurs beyond its expected level/frequency.

For an epidemic to occur  > 1

Examples of epidemic diseases includes: Avian flu/influenza, severe acute respiratory syndrome (SARS), yellow fever, and cholera

There are three types of epidemic:

  1. Common source epidemic
  2. Point source epidemic
  3. Propagated epidemic.


Common source epidemic is an epidemic in which the affected hosts acquired the disease from the same source. For example, it can result from a single common contaminated source such as food (food poisoning) or water (Legionnaire’s disease).

Figure 5. Common source epidemic curve.


Point source epidemic is an extension of common source epidemic from which the affected acquired the disease from the same source at the same point in time. Example is a disease acquired from a party.

Figure 6. Point Source Epidemic Curve.

Figure 6. Point Source Epidemic Curve.


In propagated epidemic, the frequency of the epidemic fluctuates with time and at sometimes with location. Propagated epidemic is very common with vector-borne diseases. Example of propagated epidemics includes: increase in mumps or chicken pox cases that coincides with new populations of sensitive children who arrive in class rooms each fall.


Figure 6. Propagated epidemic curve.


A pandemic disease is an epidemic that has affected more than one continent of the world. It is a series of epidemics that affects several continents or countries of the world. Examples include avian influenza/flu, SARS and HIV (AIDS) infection. While pandemic involves more than one continent of the world, epidemic involves only one continent. The curve for a pandemic disease occurrence is the same thing as that for a propagated epidemic curve. 

Figure 7. Pandemic Epidemic Curve.


In the science of epidemiology, reference is made as regards to population and not individuals. Population and not individuals is usually studied in an epidemiological investigation. This kind of study is usually done in reference to time because time causes or brings about some variations in the affected population. Epidemiology not only incorporate data on the area of infectious diseases in population groups but likewise those resulting from anatomical, deformities, genetic, constitution, metabolic dysfunction, malnutrition, occupational pursuit and the process of aging. These factors are explained briefly in below below delving into the methods of epidemiological studies.


Anatomical deformities refer to physical deformity in a person. Some deformities in people make them more prone to infections than normal people who are without deformities. For example, albinism is an anatomical deformity in people (albinos) and this makes such groups of people to be prone to certain skin cancers because of the absence of the pigment, melanin in their skin. This does not occur in non-albino persons who possess this pigment. Other anatomical deformity can be seen in blind, lame and crippled people.


Because the phenotype of a host is dependent on its genotype, selective pressure on the genetic constitution/genotype of a host can result to changes in their phenotype (i.e. their outward appearance). For example, some individuals have the “AS” genotype (abnormal) while others have “AA” genotype (normal). The “AS” genotype is as a result of selection pressure on the genetic constitution of a host. Individuals with “AS” genotype are resistant to some infections like malaria while those with “AA” genotype are more prone to malaria infection.


Metabolic dysfunction can occur in an individual when there is a deviation from the normal metabolic function of the body to an abnormal metabolic function. The kind of diet an individual eats and exposure to certain substances in the environment can bring about metabolic dysfunction. Certain infectious agents can cause metabolic dysfunction. For example, when people are infected with a particular infection such as infection with the human immunodeficiency virus (HIV), it can lead to metabolic dysfunction of the host immune system – and this predisposes the HIV-infected individual to several microbial infections. Also, most people have non-pathogenic Entamoeba histolytica infection (Amoebiasis) which can be transformed into the pathogenic E. histolytica under certain conditions like:

  • Intake of high carbohydrate (CHO) food
  • Intake of low protein food
  • Intestinal infections
  • Schistosoma haematobium infection
  • Malaria
  • Reduction in the normal bacterial flora of the gastrointestinal tract (GIT) by indiscriminate use of antibiotics.

The presence of these aforementioned factors alters or transforms the non-pathogenic E. histolytica to the pathogenic E. histolytica. When this occurs, they now start to cause an outbreak of amebiasis in that community. The normal bacterial flora in the GIT is meant to check the growth of E. histolytica, but when these conditions/factors mentioned above becomes eminent or prevalent in the individual, the non-pathogenic E. histolytica changes to the pathogenic E. histolytica, and this could result to the outbreak of amoebiasis in the population.


Malnutrition occurs when an individual is under feeding or over-feeding. It can predispose people in a population to several infections. This is because malnutrition has a way of reducing the immune system of a host thereby making them more prone to microbial infection. People should eat well balanced diet in order to counter this effect.


Certain jobs that people do predispose them to infections at a much faster rate than people who are not involved in such jobs/occupations. For example, hospital personnel are more prone to acquiring nosocomial infections than non-hospital personnel’s. Nosocomial infections are infections acquired in the hospital environment. They are hospital-acquired infections. Also, commercial sex workers are more prone to acquiring sexually transmitted disease (STDs) such as syphilis and gonorrhea than individuals who are not commercial sex workers. Farmers, builders and road construction workers are more prone to fungal infections arising from inhalation of fungal spores as a result of their job.


Susceptibility of people to infection in a population varies with age. Aging process is not only in the adults but it occurs from childhood to adulthood. Susceptibility to infection in the infants, toddlers, juvenile, young adult, adult, and the elderly varies. For example, some diseases like measles are more prone in children than in adults. Knowing the age category that is more susceptible to the infection/disease in question helps the epidemiologist in directing control measures/strategies towards the susceptible age group that is in dare need of medical intervention.


Epidemiological investigations are classified into:

  1. Observational epidemiological studies
  2. Experimental epidemiological studies.


The observational epidemiological studies are categorized into two:

  1. Description studies
  2. Analytical studies

Descriptive Epidemiological Studies examines the health status of a community based on available data. For example, morbidity and mortality statistics obtained from hospital records are used to evaluate the health status of the population under study. This data gives the epidemiologist the particular type of disease that is common or most prevalent in such community. In descriptive epidemiological studies, the pattern of disease is described according to the persons involved with respect to their age, occupation, race, education and other characteristics including the place where the disease occurs (geographical and climatic factors). All these above mentioned factors are referred to as socio-demographic parameters; and they are important parameters that aid the epidemiologists in doing their main job of disease-tracking and prevention. The time the disease occurs during the year is also considered in order to know if there are seasonal variations or other cyclical changes which may be yearly or by other intervals.

Analytical Epidemiological Studies is divided into three areas:

  1. Cross-sectional studies
  2. Case-control studies
  3. Cohort studies.

Cross-sectional studies measure the prevalence of diseases at a particular time. This study is relatively easy, economic, and useful for studying exposures that are reasonable permanent features for individuals such as economic and socio-economic status. In a sudden outbreak of a disease, a cross-sectional study with measurement of potential causative factor is often the most convenient 1st step in an investigation of the cause of the outbreak. Data’s from this study are helpful in assessing the health care need of the population.

Case-control studies can also be called a retrospective study since the investigator is looking backward from the disease to a possible cause. This study begins with the selection of cases. Here, both the cases and the control participants in the study are chosen from the same population having the disease. The controls should be carefully chosen to be representatives of the population which generated the cases. It is a relatively easy and economical type of epidemiological study.

Cohort studies begin with a group of people known as cohort (i.e. people free of the disease under investigation) who are classified into subgroups with different exposures to a potential cause of a disease. For example, 100 people who are free from the disease can be selected and screened. These individuals don’t have the disease. This 100 people can be classified into 5 sub-groups as follows. 20A, 20B, 20C, 20D and 20E. They are then exposed to different concentration of the disease cause as thus:

20A                 20B                 20C                 20D                 20E

10L                    5L                    2L                     1L                   0.5L

NOTE: Laboratory animals can be used in carrying out this study. After this, they can now be examined for the development of the disease condition. Cohort study provides the best information about the causation of a disease and the most direct measurement of the risk of developing the disease. Cohort studies often require long follow up periods since there is often a long period after exposure before the occurrence of the disease.


Experimentation is an active attempt to change a variable in a group of people. For example, the trial of a particular drug which may be new can be done or carried out using a defined population in order to determine the potency or efficacy of the drug. This type of experimentation in testing a trial drug is critical before the drug is released into the open market for public use. For example, a population of 20 people is used and they are divided into 2 groups as: Population 10A and Population 10B. The 1st group (Population 10A) is given the active drug while the 2nd group (Population 10B) is given a placebo (i.e. drug totally different from the active drug). A placebo is a tablet which appears to be a drug, but contains no medicinal substance in it. Placebos are used on control groups in tests of new drugs. The 1st group serves as the test population and they have the disease condition that the drug is supposed to act upon or cure while the 2nd group is the control group and they are free from the disease condition (even though they received the inactive part of the drug). In this way, the potency/therapeutic efficacy of the trial drug is determined.

Ethical Consideration is very important in carrying out experimental epidemiological studies. And the need for ethical consideration is aimed at:

  • Maintaining patient’s confidentiality in the course of the experimentation/research.
  • Avoiding a bridge of the patient’s fundamental human rights or infringing into it.
  • Not subjecting the patient to conditions that is detrimental to his or her health.
  • Protecting the researcher from any contamination during research.
  • Not obtaining relevant data from the patients under duress

Ethical clearance must be obtained from the ministry of health or hospital where the work is to be carried out in order to avoid bridge of patient’s confidentiality or other health policies. It is also very important that the techniques the epidemiologists or researcher is experimenting on and the laboratory procedure to be used for the study must be an already accepted procedure or criteria that must be generally acceptable by the international community as something that is workable.


  1. Randomized Control trial: This is an investigation in which subjects are randomly selected and allocated into different groups. Participants in such a study are usually divided into the test group and the control group. It is usually done to assess an experimental therapeutic agent in a given population. The results are assessed by comparing the health experience/status of the test group and that of the control group in the study. The result should be subjected to statistical analysis in order to check if there is any significant difference in the results obtained or not. If there is any significant difference, then it implies that the results obtained before was not by chance but if there is no significant statistical difference, then it imply that what happened before was by chance.

There are two categories under the randomized control trials:

  1. Single Blind trials: In single blind trials, the person administering the drug knows which patient gets the active/real drug, and those that gets the placebo. But the patient does not know whether he or she is getting the active drug or the placebo.
  2. Double Blind trial: In double blind trials, neither the investigator nor the patient knows who gets the placebo or the active drug. Placebo is not the real drug. It is a substance used in epidemiological studies; and such substances or chemicals have little or no therapeutical value. Placebos are generally inert in nature, and confer no therapeutic value to the study population taking them. Placebos only appear to be the real medical treatment; but the reverse is the case, because they are not the real drug and varies from the actual drug being tested in the study.
  3. Confounding variables: These are variables that can cause or introduce bias into the study. These variables can also introduce errors in the study or reduce its accuracy. They are very important both to descriptive and experimental studies because in every experiment it may be difficult to keep every variables at constant. There are some variables that can be controlled and some that cannot be controlled.

Controlling Confounding Factors in epidemiological studies

            The ways by which confounding factors can be controlled in an epidemiological study include:

  1. Randomization
  2. Restriction
  3. Matching
  4. Stratification
  5. Multi-variant analysis

Randomization: Randomization is simply the process of conducting or sampling at random. Random sampling is usually conducted without any specific consideration of the individuals that make up the population during sampling. Sampling at random helps the researcher to include the various representative members that make up the population under study. Here, the people that make up the population are screened without a prior knowledge of where they are coming from; what they do for a living; where they work or what they eat.

Restriction: Restriction as a means of controlling confounding variables in an epidemiological research is mainly aimed at reducing bias in the work. Because a bias may be introduced, the work is restricted to individuals who have similar characteristics within the population under study. Here, the individual’s socio-economic class (occupation, education, residence) is considered. An example of such a measure is shown in Table 1.

Table 1. Prevalence of sexually transmitted infection in Enugu (A)

Occupation No examined No (%) infected
Civil servants

Okada riders




Here, your intervention study is restricted to a particular research. Instead of saying prevalence of sexually transmitted infection (STIs) in all of the above occupation, it can be restricted as thus: Prevalence of STIs amongst Okada riders in Enugu metropolis.

Stratification involves obtaining certain socio-demographic parameters from the study population because of difficulty in defining and restricting the study to a particular group of the population. Some of these socio-demographic parameters includes: occupation, sex, religion, tribe, marital status, residential location, education level.

Table 2. Prevalence of sexually transmitted infection in Enugu (B)

Occupation                 No examined              No (%) infected

 Okada riders               100                                          50(50%)

Students                      50                                            5 (10%)

Farmers                       60                                            25(41%)

Traders                        40                                            10(25%)

Civil servants              50                                            5(10%)

Total                           300                                          95(31.7%)

For interventional purposes when using stratification model, you don’t work with the final result (31.7%) but rather with the individual results as shown in Table 2. 

Matching: In matching technique, the study population must match exactly with the control population to be used. For example, if the study population is students, then the control group in the study must also come from student population. It will be a misdemeanor to use a different control group and different test group for the study. Also, the occupation of your test population must also match the occupation of your control population if the socio-economic factor you have decided to study is the occupational aspect of the population – as it relates to the disease under investigation. Matching helps the researcher to eliminate error in the results.

Multi-variant analysis: This involves subjecting your results/findings to statistical analysis. This includes using Chi-Square (x2), regression and correlation analysis, analysis of variance (ANOVA) and T-test. In order to know if the result was by chance or not; and to test the significance of the results obtained, it is important to include multi-variant analysis in the study in order to eliminate bias. Multi-variant analysis helps in knowing the interventional measures to be used in the study. Statistical analysis is also important in controlling confounding factors and it goes ahead to prevent any possibility of doubt in the work or results obtained from the study.

In disease prevention and control; two research approaches are usually considered:

  1. Social research
  2. Scientific research – is based on experimentation carried out in the laboratory on animals or other non-human or human models to ascertain the cause of a disease outbreak in a bid to contain its further spread.

Social Research: Social research entails or involves knowing the social aspect of the community where the disease outbreak occurred. Here, the knowledge of the community about the disease outbreak is ascertained. Their attitude and practice is also ascertained. Different techniques are used in carrying out the social research and they include:

  • Identification of the socio-demographic parameters of the community. These socio-demographic parameters include the age, sex, religion, occupation, education, and the geographical and climatic factors of the disease outbreak. Social research helps in obtaining information that cannot be obtained in the laboratory. A holistic manner/approach is used in going about this, before a preventive and control strategies are put in place to contain the disease spread. In social research, a preliminary investigation is carried out in order to get a base line, so that your work will not be based on what people say. Social research has two basic areas; and they are
  1. Qualitative survey
  2. Quantitative survey

Qualitative survey involves doing interviews using an interview guide. This interview guide will contain a list of questions that the test population will be asked. In qualitative survey, there is a person-person interaction in which the investigator talks with the test population on individual basis; and the answers obtained from such a one-to-one interview are recorded by the investigator. A group discussion in which the investigator becomes the moderator can also be put up in the qualitative survey.

Quantitative survey involves the use of a structured questionnaire or pro-forma. Here, specific questions including the name, age, sex, occupation and educational status of the interviewee (i.e. the person to be interviewed) are typed in a paper and given to the person to fill and submit later. After which, the questionnaires are obtained back from the people that received them; and they are finally analyzed, interpreted and implemented in the prevention and control measures of the disease outbreak.


The principles of disease prevention and control in any disease outbreak or epidemic are usually based on the following points:

  1. Identification of the problem/preliminary preparations. This involves:
  • Definition of the problem/disease i.e. nature of the disease
  • Look at the characteristics of the disease amongst the people
  • Determine the possible etiology/etiologies of the disease
  • Constitution of an interdisciplinary project/interventional team that is headed by a project leader
  • Development of a provisional methodology in the course of a scientific meeting.
  1. Situational analysis. This involves:
  • Conduct literature review about the disease by searching the internet and the WHO data base for relevant information about the disease. Obtain mortality and morbidity statistics about the disease in health centers in the community.
  1. Development of research/interventional proposal. This involves:
  • This is of paramount importance because it gives the researcher the direction and focus as to achieving something at the end of the work. Such a document will contain the plan, strategies, aims and objectives, background of the problem, definition of the problem, definite information about the problem and the fund/budget as to go about tackling the problem.
  1. Advocacy visit/consultation with stakeholders. This involves:
  • Seek support and approval from the community where the work is to be carried out before commencing the work. You do this by visiting the person (Chief or a king) in-charge of the place and let him or her know about the work and its relevance to the community so that approval as to go ahead in doing the work will be given.
  1. This involves:
    • Institution of epidemiological survey team
    • Establishment of the etiology
    • Notification of the appropriate authorities about the disease
    • Institution of preliminary control measures.
  1. Preventive measures. This involves:
  • Establishing appropriate diagnostic facility where future or suspected cases can be handled
  • Identification and isolation/quarantine of cases
  • Contact tracing
  • Investigation of probable sources of infection
  • Immunization or mass chemotherapy
  • Health education
  • Environmental improvement and sanitation.
  1. Sustenance of interventional/control measures. This involves:
  • Involvement of the government
  • Involvement of the non-governmental organizations (NGOs)
  • Integration of interventional program into health system
  • Involvement of the local community in the interventional measures
  1. Monitoring and evaluation. This involves:
  • Apply social research
  • Screening the population for improvement on their health status
  • Assess the economic impact of the work
  1. Documentation and dissemination or research results. This involves:
  • Preparation of project report and submission to the founders of the project
  • Preparation of reports for publication in journals and newspapers or magazines.
  • Holding a press conference to discuss the implications and significance of the research outcome.


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