Glossary
chain of infection
The chain of infection is the six links an infection needs — agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host — and breaking any one of them prevents it.
The chain of infection is the six-step sequence an infection has to complete, and the organizing idea of infection control: break any one link and the infection does not occur.
The links are the infectious agent (the organism), the reservoir (where it lives and multiplies — a person, water, soil, equipment), the portal of exit (droplets, blood, urine, feces, a draining wound), the mode of transmission (how it travels), the portal of entry (the respiratory tract, a break in skin, a catheter, a mucous membrane) and the susceptible host (someone who can be infected).
Each link names its own interruption. Sterilization and disinfection attack the agent; cleaning and vector control attack the reservoir; dressings and respiratory hygiene contain the exit; hand hygiene, PPE, isolation and air handling interrupt transmission; aseptic technique and intact dressings protect the entry; vaccination and nutrition protect the host.
Transmission itself takes five recognized forms: direct contact, indirect contact through a fomite, droplet, airborne, and vector-borne, with a common vehicle such as food or water sometimes counted separately. Hand hygiene between patients breaks the chain at its most common point of transfer, which is why it is the single most effective step.
Taught in
Related terms
- Gram stain — A Gram stain is a four-reagent differential stain that sorts bacteria by cell wall: a thick wall holds the purple dye (Gram-positive), a thin wall loses it and takes the pink counterstain (Gram-negative).
- virus — A virus is genetic material wrapped in a protein coat — it has no cell, no metabolism, and can only reproduce inside a host cell.
Source: The biology guide in this course: its genetics definitions follow the National Human Genome Research Institute's Talking Glossary of Genomic and Genetic Terms, and its process descriptions follow the US National Cancer Institute's SEER Training Modules. Both are US federal government material, not subject to copyright in the United States. The microbiology entries — the bacterial cell, the Gram stain and the chain of infection — were written for this course from the infection-control guidance the biology guide's own source list names (US Centers for Disease Control and Prevention), which the genetics glossary does not cover. The definitions above are written for this course in our own words. (NHGRI Talking Glossary accessed 2026-10-26; definitions written 2026-10-27; the CDC basis for the microbiology entries — Principles of Epidemiology in Public Health Practice, third edition — accessed 2026-10-28, the date the biology guide's own source list records)