4.2 Chain of Infection
The chain of infection, also referred to as the chain of transmission, describes how an infection spreads based on these six links of transmission:
- Infectious Agent
- Reservoirs
- Portal of Exit
- Modes of Transmission
- Portal of Entry
- Susceptible Host
See Figure 4.1[1] for an illustration of the chain of infection. If any “link” in the chain of infection is removed or neutralized, transmission of infection will not occur. Health care workers must understand how an infectious agent spreads via the chain of transmission so they can break the chain and prevent the transmission of infectious disease. Routine hygienic practices, standard precautions, and transmission-based precautions are used to break the chain of transmission.
The links in the chain of infection include Infectious Agent, Reservoir, Portal of Exit, Mode of Transmission, Portal of Entry, and Susceptible Host[2]:
- Infectious Agent: Microorganisms, such as bacteria, viruses, fungi, or parasites, that can cause infectious disease.
- Reservoir: The host in which infectious agents live, grow, and multiply. Humans, animals, and the environment can be reservoirs. Sometimes a person may carry an infectious agent but is not symptomatic or ill. This is referred to as being colonized, and the person is referred to as a carrier. For example, many health care workers carry methicillin-resistant Staphylococcus aureus (MRSA) bacteria in their noses but are not symptomatic.
- Portal of Exit: The route by which an infectious agent escapes or leaves the reservoir. In humans, the portal of exit is typically a mucous membrane or other opening in the skin. For example, pathogens that cause respiratory diseases usually escape through a person’s nose or mouth.
- Mode of Transmission: The way in which an infectious agent travels to other people and places because they cannot travel on their own. Modes of transmission include contact, droplet, or airborne transmission. For example, touching sheets with drainage from one person’s infected wound and then touching another person without washing one’s hands is an example of contact transmission of an infectious agent. Examples of droplet or airborne transmission are coughing and sneezing, depending on the size of the microorganism.
- Portal of Entry: The route by which an infectious agent enters a new host (i.e., the reverse of the portal of exit). For example, mucous membranes, skin breakdown, and artificial openings in the skin created for the insertion of medical equipment (such as intravenous lines) are at high risk for infection because they provide an open path for microorganisms to enter the body. Tubes inserted into mucous membranes, such as a urinary catheter, also facilitate the entrance of microorganisms into the body. A person’s immune system fights against infectious organisms that have entered the body through the use of nonspecific and specific defenses. Read more about defenses against microorganisms in the “Defenses