The Triangle of Epidemiology

The triangle of epidemiology can also be called epidemiological triad. 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 among 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 among 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 among the susceptible host factors, environment and the pathogen might eventually result to an epidemic, endemic disease or even a pandemic.

Figure 1: Epidemiologic triad.


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

Stedman’s medical dictionary, 27th edition. Philadelphia: Lippincott, Williams and Wilkins.

Songer T (2005). Study designs in epidemiologic research. Supercourse, ( (Accesed May 2103).

Singleton P and Sainsbury D (1995). Dictionary of microbiology and molecular biology, 3d ed. New York: John Wiley and Sons.

Taylor LH, Latham SM, Woolhouse ME (2001). Risk factors for disease emergence. Philos Trans R Soc Lond B Biol Sci, 356:983–989.

Salyers A.A and Whitt D.D (2001). Microbiology: diversity, disease, and the environment. Fitzgerald Science Press Inc. Maryland, USA.

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

Rothman K.J and Greenland S (1998). Modern epidemiology, 2nd edition. Philadelphia: Lippincott-Raven.

Porta M (2008). A dictionary of epidemiology. 5th edition. New York: Oxford University Press.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

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

Mandell G.L., Bennett J.E and Dolin R (2000). Principles and practice of infectious diseases, 5th edition. New York: Churchill Livingstone.

Merill R.M (2012). Introduction to Epidemiology. Sixth edition. Jones and Bartleh Learning,

Molyneux, D.H., D.R. Hopkins, and N. Zagaria (2004) Disease eradication, elimination and control: the need for accurate and consistent usage. Trends Parasitol, 20(8):347-51.

MacMahon   B.,   Trichopoulos   D (1996). Epidemiology Principles and Methods.   2nd ed. Boston, MA: Little, Brown and Company. USA.

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

Aschengrau, A., & G. R. Seage III. (2009). Essentials of Epidemiology in Public Health.  Boston:  Jones and Bartlett Publishers.

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