Public and Military Health Posters for Contagious and Infectious Disease

In everyday speech, and even in many news reports, the terms “contagious“ and ”infectious“ are often used interchangeably. In epidemiology (the study of how diseases spread) and most other scientific fields, however, they have distinct definitions. All contagious diseases are infectious, but not all infectious diseases are contagious.

Infectious diseases:

  • Are caused by “infective agents” – that is, bacteria, viruses, fungi, parasites, or prions – which are non-self organisms.
  • Cause clinically evident disease.
  • Not caused by immune dysfunction, non-infected injury, or psychological conditions.
  • Not caused by bodily reactions to chemicals or poisons not secreted by infective agents.
  • Transmitted in many, many ways, but generally originate outside of the infected host. An exception is in immune-compromised patients who become infected by commensal organisms.

Contagious diseases:

  • Are infectious diseases transmitted from person-to-person, with no special agent or vector required.
  • Can be spread via airborne droplets, other bodily secretions, or fomites (any object or substance capable of carrying infectious organisms, such as clothing, money, doorknobs, or stethoscopes).
  • Are the cause of most epidemics (a notable exception is the Black Plague, which probably was caught through flea vectors).
  • Spread can be controlled by quarantine and isolation.

Another context in which “infectious” and “contagious” are used is to describe something as highly infectious or highly contagious. 

Highly infectious:

  • Symptomatic disease can be caused by a very low number of infectious agents being introduced into the body.
  • Some highly infectious agents (such as ebola), can be caused by a very low number of pathogens, but can only cause infection when introduced into the body in a specific manner – for example, ebola does not cause infection when inhaled, but a tiny droplet of infected bodily secretion landing on an open wound can cause disease.

Highly contagious:

  • Generally refers to the ability of the pathogen to survive outside of the host, and the number of ways it can be transmitted.
  • Can be spread through airborne droplets.

To use the ebola example, even though it can’t be caught through airborne droplets, it can be caught through fomites, dead bodies, sexual intercourse, and contact with almost any bodily fluids. Because it’s not airborne, however, it’s considered highly infectious but not highly contagious, at least by virologists.

However, for practical use, because it is so infectious, and has many other modes of transmission, it’s often called “highly contagious” in the media.

Posters from National Archive of Medical History’s Otis Archives

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