Chain of Infection: Understanding Disease Transmission

Chain of Infection

The chain of infection is a model used to understand the transmission of infectious diseases. It consists of six elements: reservoir, mode of transmission, susceptible host, portal of entry, portal of exit, and infectious agent.

Reservoir

• A reservoir is the natural habitat of an infectious agent
• Can be a person, animal, or environment
• Examples:
  • Humans (e.g., COVID-19)
  • Animals (e.g., mosquitoes for malaria)
  • Environment (e.g., contaminated water for cholera)

Mode of Transmission

• The way an infectious agent is transmitted from the reservoir to a 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., mosquitoes, ticks)

Susceptible Host

• A person or animal that is not immune to an infectious agent
• Factors that increase susceptibility:
  • Age (e.g., young, elderly)
  • Underlying health conditions (e.g., diabetes, HIV)
  • Weakened immune system (e.g., cancer, chemotherapy)

Portal of Entry

• The route by which an infectious agent enters a susceptible host
• Examples:
  • Respiratory tract (e.g., influenza)
  • Gastrointestinal tract (e.g., food poisoning)
  • Skin (e.g., cuts, wounds)
  • Mucous membranes (e.g., eyes, nose, mouth)

Portal of Exit

• The route by which an infectious agent leaves a reservoir
• Examples:
  • Respiratory tract (e.g., coughing, sneezing)
  • Gastrointestinal tract (e.g., diarrhea, vomiting)
  • Skin (e.g., wounds, skin lesions)
  • Mucous membranes (e.g., eyes, nose, mouth)