What is the Chain of Infection?

The chain of infection is a way of gathering the information needed to interrupt or prevent an epidemic. Each of the links in the chain must be favorable to the organism for the epidemic to continue. Breaking any link in the chain can disrupt the epidemic. Which link it is most effective to target will depend on the organism.

Links in the Chain:

• The Organism: What is the organism? Bacteria, virus, protist, parasite, or fungi? The type of organism informs you of the types of disinfectants, antiseptics and antimicrobials to use. Is it aerobic or anaerobic? What are its virulence factors? Toxin production in particular effect the course of the infection. What is its target host tissue?

• The Reservoir: Where do you find the organism in between outbreaks? What is the continual source of the infection? A reservoir can be environmental, the hospital setting or the water supply, or in a living organism, a rodent, bird or even snail. Humans are the only reservoir for many human pathogens.

• Portal of Exit: How does the organism leave the reservoir? Does it leave in feces, blood or mucus; in contaminated water; or in the blood meal of an insect.

• Transmission: How is the organism transmitted from one host to the next host? Does it need a living vector like a mosquito or flea? Some organisms like malaria have complicated life cycles involving more than one species. Can it be passed human to human? When passed human to human, it is transmitted by respiratory droplets, blood contact, semen or other secretions? Is it transmitted on the hands of health care workers or the hospital ventilation system? Hand-to-mouth is a common mode for gastrointestinal pathogens.

• Portal of Entry: How does the organism enter the body? Does it come through inhalation, a break in the skin or mucus membrane, an insect bite, contaminated food? The portals of entry would be through the nose, skin, or mouth. Portal of entry tells you what type of personal protective equipment (PPE) to use to keep health care workers, family and visitors safe.

• Vulnerable Populations: Who is most vulnerable to this organism? Common vulnerable populations are the very young and the very old, and  the immune suppressed (due to genetics, transplant drugs, malnutrition, or viral infection like HIV). Occupational exposure should be considered.  For many human pathogens, all of the non-immune are vulnerable. The non-immune are everyone who has not been previously exposed and generated a specific immune response to the pathogen. Previous exposure comes from either a previous infection or vaccination, or rarely with infection of a similar species that gives cross-immunity.