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Chain of Infection

Development of an infection is dependent upon an uninterrupted process, referred to as – chain of infection. This process is dependent upon the following elements:

  • pathogens in sufficient numbers
  • a reservoir for pathogen growth
  • a portal of exit from the reservoir
  • a mode of transmission
  • a portal of entry to the host, and
  • a susceptible host

This chain of infection can be broken by infection control measures implemented by health care workers. These measures either terminate the threat or block the mechanism leading to the next link in the chain. An infection may be stopped at any point in the chain of infection.

How does the Chain of Infection Work?

A reservoir is any person, animal, arthropod, plant, soil, or substance (or combination of these) in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that it can be transmitted to a susceptible host. There can be Animate reservoirs include people, insects, birds, and other animals. Inanimate reservoirs include soil, water, food, feces, intravenous fluid and equipment.

The pathogen then leaves the reservoir via the portal of exit. This is the site from where micro-organisms leave one host to enter another host and cause disease or infection. For example, a micro-organism may leave the reservoir through the nose or mouth when someone sneezes or coughs, or in feces. Ports of exit include: Upper respiratory tract, Gastrointestinal tract, Blood, Urogenital tract, Skin and mucous membranes.

Once a pathogen has exited the reservoir, it needs a mode of transmission to the host through a portal of entry. Transmission can be by direct or indirect contact or through airborne transmission.

Direct contact is person-to-person transmission of pathogens through touching, biting, kissing, or sexual intercourse. Microorganisms can also be expelled from the body by coughing, sneezing or talking. The organisms travel in droplets over less than 1 meter in distance and are inhaled by a susceptible host.

Indirect contact includes both vehicle-borne and vector-borne contact. A vehicle is an inanimate go-between, an intermediary between the portal of exit from the reservoir and the portal of entry to the host. Inanimate objects such as handkerchiefs and tissues, soiled laundry, and surgical instruments and dressings are common vehicles that can transmit infection.

The infectious agents then enter the body through a portal of entry. After an infectious agent gets inside the body it has to multiply in order to cause the disease. In some hosts, infection leads to the disease developing, but in others it does not. Individuals who are likely to develop a communicable disease after exposure are called susceptible hosts. This is due to a low level of immunity within the more susceptible individuals. Immunity refers to the resistance of an individual to communicable diseases, because their white blood cells and antibodies (defensive proteins) are able to fight the infectious agents successfully. Low levels of immunity could be due to:

  • diseases which suppress immunity
  • individuals receiving chemotherapy
  • poorly developed or immature immunity, as in very young children
  • not being vaccinated
  • poor nutritional status
  • pregnancy

Breaking the Chain of Infection:

The chain of infection can be broken and infection avoided or disrupted by breaking any one of the links within the chain. For instance, healthcare workers may avoid the transmission of pathogens by using standard precautions. Immunization against common pathogens such as influenza, measles and pertussis will thwart may viral agents. Additional examples follow:

  • Correct hand hygiene is the most important basic practice for the prevention of transmission of pathogens
  • Maintenance of skin and mucous membrane integrity (caregiver and client)
  • Apply standard precautions when handling excreta, exudate, and soiled linen.
  • Cleaning, disinfection, and sterilization of reusable instruments and equipment
  • Maintenance of aseptic techniques, use of sterile equipment or single use equipment
  • Elimination of sources of infection (reservoirs)
  • Early diagnosis of infectious diseases (decreased number of pathogens for transmission)
  • Isolation of persons suffering from infectious diseases
  • Appropriate handling and disposal of body secretions – vomitus, feces, sputum, blood and body fluids
  • Cover nose and mouth when sneezing or coughing, and dispose of facial tissues after use.
  • Immunization against infectious diseases
  • Collection and disposal of waste in communities, adequate drainage and sewerage facilities



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