Chain of infection: agent, host, reservoir, portals of exit, mode of transmission, portals of entry

Chain of infection: agent, host, reservoir, portals of exit, mode of transmission, portals of entry

Chain of Infection

Infectious Agent

  • The infectious agent is the microorganism - such as a virus, bacteria, fungi or parasite - that causes the disease.

  • These agents can vary greatly in size and shape, from microscopic bacteria to larger parasites.

  • In order to cause disease, they need to be able to multiply in the host’s body, evade the host’s immune system, and cause damage to the host’s cells or tissues.

Reservoir

  • The reservoir is where the infectious agent lives and multiplies. This could be a human, an animal, or an environment such as water or soil.

  • Reservoirs are crucial in the lifecycle of the infectious agent as they provide a supportive environment for growth and multiplication.

  • Some diseases have only one specific type of reservoir, while others might have multiple potential reservoirs.

Portals of Exit

  • Portals of exit are the ways in which the infectious agent leaves the reservoir. For example, through a person’s nose when they sneeze, in faecal matter, or in blood from a wound.

  • Different infectious agents often have different preferred portals of exit. For instance, respiratory pathogens usually exit through the mouth or nose, while gastrointestinal pathogens leave through faecal matter.

Mode of Transmission

  • The mode of transmission refers to how the infectious agent travels from the reservoir to a new host.

  • There are several different modes of transmission, including direct contact, indirect contact through a vector or a fomite, airborne transmission, and food or waterborne transmission.

Portals of Entry

  • Portals of entry are the ways in which the infectious agent can enter a new host.

  • Examples include the mouth, nose, and breaks in the skin. Some pathogens have a preferred portal of entry that will allow them to more efficiently establish an infection.

Susceptible Host

  • The susceptible host is an individual who is not immune or resistant to the infectious agent.

  • Factors that might make a host more susceptible include age, health status, immune function, and whether they have been vaccinated against the disease.

Key Themes

  • The key theme across the chain of infection is that for an infection to occur, all parts of the chain need to be present and connected.

  • Interrupting one part of the chain, such as through vaccination, appropriate hygiene practices, or pest control, can prevent the spread of disease.