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Infectious disease results from the interaction of an agent, a host, and an environment. Most of these diseases follow a typical chain of infection that ends with an infected host. Understanding the chain of infection can help with both the prevention and treatment of infectious diseases.

Definition: The Chain of Infection

How do viruses pass from one person to the next? When one child has a cold at school, why is the rest of the class likely to come down with one the next week? Let’s explore how infection gets from one place to another, or the chain of infection.

chain of infection

The chain of infection, if we think of it as an actual chain, is made up of six different links: pathogen (infectious agent), reservoir, portal of exit, means of transmission, portal of entry, and the new host. Each link has a unique role in the chain, and each can be interrupted, or broken, through various means.

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The Six Links

The first link is the pathogen itself. This is the disease-causing organism.

For many illnesses and diseases this is a virus or bacterium. In order to break this link, various methods can be used, including the pasteurization of milk, the chlorination of drinking water, or the use of disinfectants.The second link is the reservoir. This is the natural environment that the pathogen requires for survival. Reservoirs can be a person, an animal, or an environmental component, such as soil or water. This link can be broken through medical treatment and testing, insect and rodent eradication, or quarantine.

The third link is the portal of exit. This link is needed for the pathogen to leave the reservoir. If the reservoir is a human, then the portal of exit may be saliva, mucous membranes, feces, blood, or nose or throat discharges. By using barrier methods, such as condoms or masks, or covering the mouth while coughing, this link can be broken.The fourth link is the means of transmission.

The pathogen can be transmitted either directly or indirectly. Direct transmission requires close association with the infected host, but not necessarily physical contact. Indirect transmission requires a vector, such as an animal or insect. The link can be broken through hand washing, safe sex practices, or avoiding contact with infected individuals.Link number five is the portal of entry. Entry of the pathogen can take place in one of three ways: penetration, inhalation, or ingestion.

The level and severity of an infection may depend on the depth of penetration. Similar to the portal of exit, barrier methods, such as condoms or masks, can be used to break this link along with other methods, such as insect repellants.The final link is the new host. Once in the new host, various factors influence the severity of infection, including the strength of the immune system and the reproductive rate of the pathogen. Immunization, health promotion, and medical treatment can be used to break this link in the chain.

Example of a Chain of Infection

An example of illness resulting from the chain of infection is the common cold.

In this case, the pathogen is often referred to as rhinovirus. The reservoir is another person carrying this virus, who then propels the virus into the air via a portal of exit, such as a cough or sneeze. The route of transmission is direct to the new host, which takes place through inhalation (the portal of entry) of the virus.

Control Measures

According to the Center for Disease Control and Prevention, knowledge of the portals of exit, entry, and modes of transmission provides a basis for determining appropriate control measures. In general, control measures are directed against the segment in the infection chain that is most susceptible to intervention, unless practical issues dictate otherwise.

Potential interventions include controlling or eliminating the agent at the source of transmission, protecting portals of entry, and increasing the host’s defenses.

Lesson Summary

An illness begins with the chain of infection. The first step in this chain is a pathogen (such as a virus or bacterium) that lives in a reservoir. The reservoir may be a human, an animal, or part of the environment.

The pathogen exits its reservoir and is transmitted toward the new host. One method of transmission from a human reservoir is coughing. The pathogen then enters the new host. It may be inhaled or ingested.

It may also penetrate through a mucous membrane or a wound.Disease control depends upon breaking one of the links in this chain. Understanding the pathogen, its portals of exit and entry, and its transmission is important. This understanding can help people identify the most susceptible link and focus prevention efforts on breaking it.

Key Terms and Definitions

Chain of infection
  • Chain of infection: the steps required for an illness to pass from one person to another
  • Pathogen:the disease-causing organism
  • Reservoir: the natural environment that the pathogen requires to survive
  • Portal of exit: what is needed for the pathogen to leave the reservoir
  • Means of transmission: how the pathogen passes directly or indirectly from one reservoir to the next
  • Portal of entry: the way in which the pathogen enters the new host, either through inhalation, penetration or ingestion
  • New host: the pathogen’s new reservoir

Learning Outcomes

When the video lesson reaches its conclusion, assess your capacity to:

  • Enumerate the six steps of the chain of infection
  • Cite an example of a chain of infection
  • Consider the control measures that can be used to disrupt the chain

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