The Six Links in the Chain of Infection

What is the Cycle of Infection?

The cycle of infection describes the six steps or links required for an infectious agent to spread from one person or animal to another. Interrupting any link in this chain prevents the spread of infection.

1. Infectious Agent: This is the pathogen—the germ, such as a bacteria, virus, fungus, or parasite—that causes the disease. Examples include Salmonella bacteria (food poisoning), the influenza virus (the flu), and Plasmodium parasites (malaria).

2. Reservoir: This is where the infectious agent lives and multiplies. Reservoirs can be humans (carrying a disease), animals (zoonoses), or the environment (soil, water). For example, E. coli bacteria can reside in the intestines of humans and animals, serving as a reservoir.

3. Portal of Exit: This is how the infectious agent leaves the reservoir. Examples include bodily fluids (saliva, blood, feces, urine), respiratory droplets (coughing, sneezing), or skin lesions (open wounds). The influenza virus, for example, exits the body via respiratory droplets.

4. Mode of Transmission: This is how the infectious agent travels from the reservoir to a new host. Common modes include:

   • Direct contact: Touching an infected person or animal.
   • Indirect contact: Touching a contaminated object (fomites), such as a doorknob.
   • Airborne transmission: Inhaling airborne droplets containing the infectious agent.
   • Vector transmission: Transmission through an intermediate host, like a mosquito carrying malaria.

5. Portal of Entry: This is how the infectious agent enters a new host. This often mirrors the portal of exit; for example, the influenza virus enters through the respiratory tract. Other portals include mucous membranes, skin breaks, and catheters.

6. Susceptible Host: This is an individual who is vulnerable to infection because their immune system is weakened or they lack immunity to the specific pathogen. Factors influencing susceptibility include age, underlying health conditions, and vaccination status. A person with a weakened immune system is more susceptible to various infections.

By understanding and breaking the chain of infection at any of these six links, we can effectively control and prevent the spread of infectious diseases. This is crucial in healthcare settings and public health initiatives.