Understanding the Chain of Infection

The Chain of Infection

The ongoing COVID-19 pandemic has upended nearly every aspect of life, quickly challenging past assumptions and future certainties. Emerging infectious diseases such as COVID-19 can rapidly spread across national boundaries and communities, challenging the ability of public health systems to prevent and control the spread of the disease, especially in resource-limited countries and regions.

The spread of an infectious disease within a community is referred to as the “chain of infection.” This is a series of interconnected steps that describe how a pathogen moves about. Infection control and contact tracing are meant to break the links of the chain, which prevents a pathogen from spreading.

The spread of infection can be described as a chain with six links:

1.      Infectious agent

2.      Reservoir

3.      Portal of exit

4.      Mode of transmission

5.      Portal of entry

6.      Susceptible host

Figure 1

The Chain of Infection

The first link in the chain is the infectious agent (Figure 1), which is the harmful germ or pathogen that causes the infectious illness or disease. Examples of pathogens may include bacteria, viruses, and fungi.

The second link is the reservoir or source, which is where the pathogen typically resides, grows, and multiplies. This can be in or on a person or an animal. Some replicate in environmental reservoirs such as water or soil.

The third link is a portal of exit, and this is how pathogens leave their natural reservoir. For example, pathogens that cause respiratory diseases typically escape through the respiratory tract when a person coughs or sneezes. Pathogens may also leave the body of the infected person by means of bodily secretions (e.g., stool and vomit) or broken skin.

The mode of transmission is the fourth link in the chain. It refers to how the pathogen is passed from one person to another. For instance, contact is the most common route of transmission for workplace pathogens, which can be transmitted by direct contact through touching or via indirect contact from suspended air particles or inanimate objects. Pathogens such as those that cause influenza and chicken pox can remain in the air for long periods of time and subsequently inhaled by others.

The fifth link is the portal of entry where pathogens enter the body of the potential host. Pathogens can enter through inhalation, ingestion, broken skin, or through the eyes, nose, and mouth. Pathogens can also be introduced into the body via medical procedures.

The six and final link of the chain is the susceptible host. The susceptible host is the individual that the pathogen moves to. The risk of the person becoming infected depends on various factors such as their general health and the strength of their immune system.

Breaking the Chain

Preventing infection means breaking the links in the chain so that an infection cannot spread. Some links are easier to break than others. Infection control measures are generally directed at the link in the infection chain that is most susceptible to intervention, including controlling or eliminating the agent at the source of transmission, protecting portals of entry, or increasing the host’s defenses.

Standard precautions are actions that should be taken in every situation to reduce the risk of infection. These include the practice of good hand hygiene, safe disposal of waste, safe management of laundry, and the correct use of personal protective equipment.


ATrain Education. (n.d.). ATrain education. 2. Understanding the Chain of Infection. https://www.atrainceu.com/content/2-understanding-chain-infection.

Centers for Disease Control and Prevention. (2012, May 18). Principles of epidemiology. Centers for Disease Control and Prevention. https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section10.html.

Morens, D. M., & Fauci, A. S. (2020, August 15). Emerging Pandemic Diseases: How We Got to COVID-19. Cell. https://www.cell.com/cell/fulltext/S0092-8674(20)31012-6?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0092867420310126%3Fshowall%3Dtrue.

MUN Global. (2020, November 12). What is the chain of infection and how to break it. https://munglobal.com.au/resources/knowledge-base/pathogens/what-is-the-chain-of-infection/.

A guest post by