COMMUNICABLE DISEASES: CHAIN OF INFECTION

 The chain of infection is a set of 6 intertwined links that allow for communicable diseases to spread.

Each step of the chain is required to effectively transmit infectious illness.


  1. Infectious agent (pathogen)
  2. Reservoir (the normal location of the pathogen)
  3. Portal of exit from the reservoir
  4. Mode of transmission
  5. Portal of entry into a host
  6. Susceptible host
Infectious Agents

Infectious agents (pathogens) include not only bacteria but also viruses, fungi, and parasites. The virulence of these pathogens depends on their number, their potency, their ability to enter and survive in the body, and the susceptibility of the host. For example, the smallpox virus is particularly virulent, infecting almost all people exposed. In contrast, the tuberculosis bacillus infects only a small number of people, usually people with weakened immune function, or those who are undernourished and living in crowded conditions.

Reservoir
A reservoir is any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agent normally lives and multiplies.

Portal of Exit
Portals of exit is the means by which a pathogen exits from a reservoir. For a human reservoir, the portal of exit can include blood, respiratory secretions, and anything exiting from the gastrointestinal or urinary tracts

Mode of transmission
This is the way through which the pathogen transmit itself into the host. It can be through;
✓ Direct transmission e.g physical contact, airborne or water borne, or;
✓ indirect transmission e.g vector borne or vehicle borne.

Portal of entry into the host
This is any route that a pathogen uses to enter the body (host).

Examples include:

  • Inhalation (via the respiratory tract)
  • Absorption (via mucous membranes such as the eyes)
  • Ingestion (via the gastrointestinal tract)
  • Inoculation (as the result of an inoculation injury)
  • Introduction (via the insertion of medical devices)
Susceptible Host
The last link in the chain of infection is the susceptible host. This is the organism (e.g., You or your resident!) that will feel the effects of the infectious disease that has traveled through the chain of infection.

How susceptible a host may be, depends on many factors:

  • Age: The very young or very old are usually more susceptible.

  • Health status:Malnourished, dehydrated, or otherwise unhealthy persons are more at risk

  • Medication usage:Immune suppressing drugs allow pathogens to take hold more freely

  • General resistance factors: Intact mucous membranes and skin, and robust cough and sneeze reflexes help defend against invading pathogens


Credits to Atrain and Nurse aide

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