Cycle of infection

The cycle of infection — also called the chain of infection — describes the sequence of conditions a pathogen needs to spread from one host to another. It has six links: an infectious agent (bacteria, virus, fungus, or parasite), a reservoir where the agent lives and multiplies, a portal of exit, a mode of transmission, a portal of entry, and a susceptible host.

Each link is concrete. The reservoir might be an infected person, an animal, contaminated water, or a surface. Portals of exit include respiratory droplets, blood, and body fluids leaving the reservoir. Transmission can be direct (touch, droplet spray) or indirect (contaminated instruments, airborne particles, vectors like mosquitoes). The pathogen then enters a new host through a portal of entry — broken skin, mucous membranes, the respiratory or digestive tract — and infection takes hold only if that host is susceptible, which is why age, immune status, and vaccination matter.

The model's power is practical: breaking any single link stops transmission. Hand hygiene interrupts indirect transmission, disinfection and sterilization eliminate reservoirs on equipment, personal protective equipment blocks portals of exit and entry, and immunization reduces host susceptibility. Standard precautions in healthcare settings exist precisely to attack multiple links at once.

The CCMA (Certified Clinical Medical Assistant) exam tests the cycle of infection within infection control — expect questions asking you to name the six links in order, classify a scenario by its mode of transmission, and match a control measure to the link it breaks.