Chain of Infection

Chain of Infection

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:

Reservoirs

• 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)

Portal of Exit

• 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)

Mode of Transmission

• 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)

Susceptible Host

• 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

Portal of Entry

• 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)

Chain of Infection

• 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

Reservoirs

• 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

Portal of Exit

• 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

Mode of Transmission

• 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

Susceptible Host

• 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

Portal of Entry

• 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