Written by MicroDok

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.


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