THE TRIANGLE OF EPIDEMIOLOGY (Epidemiological triad)

by DrChika

The characteristics of a disease are largely dependent on the relationship between the environment, the disease causing microorganism (pathogen) and a susceptible host. Health; it must be noted is a state of equilibrium or balance between susceptible host (the individual) and the agent (pathogenic microorganism).

The features of host, environment, and agent (disease cause) are referred to as the “epidemiologic triad” and they are always taken into consideration when trying to unravel and contain the outbreak of an infection/disease within a particular community/population. The environment brings the susceptible host and the pathogen together.

Each of these contending variables (the environment, susceptible host and the pathogen) influences one another and the poor health or good health of the susceptible host is directly or indirectly dependent on the maintenance of a balance amongst the trio (i.e. the pathogen, susceptible host and the environment).

These factors can either be intrinsic or extrinsic depending on the one being investigated. The environment, pathogen and a susceptible host are always interacting amongst each other in an array of multifaceted ways which eventually results to the production or non-production of a disease/infection.

These factors are examined in the course of an epidemiological study (e.g. analytical study) in order to decipher the cause of a particular disease. An imbalance amongst the susceptible host factors, environment and the pathogen (Figure 1) might eventually result to an epidemic, endemic disease or even a pandemic.  

Figure 1. Epidemiologic triad.


The environmental factors are those factors which create the chance for exposure of susceptible individuals to a particular infection in a given community. These environmental factors that influence the distribution of disease or infection in a given area include:

  • Biological factors e.g. mosquitoes, houseflies and other vectors that help in transmitting the pathogen from the environment or other sources to a susceptible host.
  • Physical factors e.g. climatic conditions, surroundings of homes, water, food and hospitals, and the soil.
  • Socio-economic factors e.g. personal hygiene, public sanitation, overcrowding, availability of public conveniences such as toilets, and availability of health clinics.     


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1 comment

Edeh Chijioke March 4, 2023 - 6:56 pm

Quite insightful!


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