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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 (Figure 1). 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 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:

  1. 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.
  2. Physical factors e.g. climatic conditions, surroundings of homes, water, food and hospitals, and the soil.
  3. Socio-economic factors e.g. personal hygiene, public sanitation, overcrowding, availability of public conveniences such as toilets, and availability of health clinics.     


Once a disease-causing microorganism (pathogen) gains entry into the body of a susceptible host, the pathogen will adhere or attach itself to specific receptors on the cell surfaces of the host. Upon successful invasion and binding, the microorganism will begin to release chemicals that will eventually attack the host cells.The pathogen (agent) factors includes all disease causing microorganisms that invade and colonize the susceptible host and release within them many variety of virulent factors that eventually cause the host to become sick and even bed-ridden in some cases depending on the severity of the infection/disease. These microorganisms release toxins and virulent factors that attack and damage the tissues and cells of the host including the host’s immune system (in some cases e.g. HIV infection) that is supposed to stop the external aggression of the pathogens. These pathogens including other chemical and physical factors are normally necessary in sufficient amount in order to initiate a disease or infection state in the susceptible host. Pathogen factors influence the chances for the manifestation of a disease in an individual. Pathogen factors that influence a disease include:

  1. Pathogens e.g. bacteria, viruses, protozoa, and fungi.
  2. Chemical factors e.g. mutagens
  3. Physical factors e.g. ultraviolet (UV) radiation


Susceptible host factors are those characteristics of an individual that predisposes him or her to the contracting or acquisition or development of a disease. These factors are usually inherent in the susceptible host and, they determine the susceptibility or exposure of a person to a given infection within a defined population. Some susceptible host factors that influence an individual’s susceptibility to a given infection include:

  1. Personal traits e.g. race, skin colour, age, sex, immunologic status, genetic make-up.
  2. Personal habits and lifestyle e.g. drinking, smoking, eating habits, sexual practices.
  3. Socio-economic factors e.g. occupation, nutrition, place of living, place of work.


Aschengrau A and Seage G.R (2013). Essentials of Epidemiology in Public Health. Third edition. Jones and Bartleh Learning,

Gordis L (2013). Epidemiology. Fifth edition. Saunders Publishers, USA.

Nelson K.E and Williams C (2013). Infectious Disease Epidemiology: Theory and Practice. Third edition. Jones and Bartleh Learning. 

Riedel S (2004). Biological warfare and bioterrorism: a historical overview. BUMC Proceedings, 17:400-406. 

Rothman K.J, Greenland S and Lash T.L (2011). Modern Epidemiology. Third edition. Lippincott Williams and Wilkins, Philadelphia, PA, USA.

Schneider M.J (2011). Introduction to Public Health. Third edition. Jones and Bartlett Publishers, Sudbury, Massachusetts, USA.

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