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 / FUNCTIONAL USE
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:
- Non-living environmental factors
- Living environmental factors
The non-living environmental factors include:
- High or low temperature.
- Rainfall / humidity
- Windfall / direction
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
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: 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.
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