Chain of Infection

Study Notes

The chain of infection is a model that describes the sequence of events that must occur for an infection to spread. It consists of six elements:

A reservoir is the natural habitat of an infectious agent
Examples:
- Humans (e.g., carriers of MRSA)
- Animals (e.g., birds with avian influenza)
- Environment (e.g., water contaminated with Legionella)
- Fomites (e.g., contaminated surfaces, medical equipment)

The portal of exit is the route by which the infectious agent leaves the reservoir
Examples:
- Respiratory droplets (e.g., coughing, sneezing)
- Fecal-oral route (e.g., contaminated food, water)
- Skin lesions (e.g., HSV)
- Blood (e.g., needlestick injury)

The mode of transmission is the way in which the infectious agent is transferred from the reservoir to the susceptible host
Examples:
- Direct contact (e.g., touching, shaking hands)
- Indirect contact (e.g., contaminated surfaces, fomites)
- Airborne transmission (e.g., respiratory droplets)
- Vector-borne transmission (e.g., mosquito-borne diseases)

A susceptible host is an individual who is at risk of infection due to a lack of immunity or compromised immune system
Factors that increase susceptibility:
- Age (e.g., young children, older adults)
- Underlying medical conditions (e.g., diabetes, HIV/AIDS)
- Weakened immune system (e.g., chemotherapy, immunosuppressive therapy)
- Poor nutrition and hygiene

The portal of entry is the route by which the infectious agent enters the susceptible host
Examples:
- Respiratory tract (e.g., inhalation of respiratory droplets)
- Gastrointestinal tract (e.g., ingestion of contaminated food or water)
- Skin (e.g., cuts, abrasions, insect bites)
- Mucous membranes (e.g., eyes, nose, mouth)

The chain of infection is a model that describes the sequence of events necessary for an infection to spread
It consists of six essential elements: reservoir, portal of exit, mode of transmission, susceptible host, portal of entry, and infectious agent

A reservoir is the natural habitat of an infectious agent
Examples of reservoirs include humans, animals, environment, and fomites
Humans can be reservoirs as carriers of infectious agents, such as MRSA
Animals can be reservoirs, such as birds with avian influenza
Environment can be a reservoir, such as water contaminated with Legionella
Fomites can be reservoirs, including contaminated surfaces and medical equipment

The portal of exit is the route by which the infectious agent leaves the reservoir
Examples of portals of exit include respiratory droplets, fecal-oral route, skin lesions, and blood
Respiratory droplets can be a portal of exit through coughing or sneezing
Fecal-oral route can be a portal of exit through contaminated food or water
Skin lesions can be a portal of exit, such as HSV
Blood can be a portal of exit, such as through a needlestick injury

The mode of transmission is the way in which the infectious agent is transferred from the reservoir to the susceptible host
Examples of modes of transmission include direct contact, indirect contact, airborne transmission, and vector-borne transmission
Direct contact involves touching, shaking hands, or other forms of direct contact
Indirect contact involves contaminated surfaces, fomites, or other forms of indirect contact
Airborne transmission occurs through respiratory droplets
Vector-borne transmission occurs through mosquito-borne diseases or other vectors

A susceptible host is an individual who is at risk of infection due to a lack of immunity or compromised immune system
Factors that increase susceptibility include age, underlying medical conditions, weakened immune system, and poor nutrition and hygiene
Age is a factor, as young children and older adults are more susceptible to infection
Underlying medical conditions, such as diabetes or HIV/AIDS, can increase susceptibility
Weakened immune system, such as through chemotherapy or immunosuppressive therapy, can increase susceptibility
Poor nutrition and hygiene can increase susceptibility to infection

The portal of entry is the route by which the infectious agent enters the susceptible host
Examples of portals of entry include respiratory tract, gastrointestinal tract, skin, and mucous membranes
Respiratory tract can be a portal of entry through inhalation of respiratory droplets
Gastrointestinal tract can be a portal of entry through ingestion of contaminated food or water
Skin can be a portal of entry through cuts, abrasions, or insect bites
Mucous membranes can be a portal of entry through eyes, nose, or mouth