Which of the following is the first link in the chain of infection?

Question:  Which of the following is the first link in the chain of infection?

A   Susceptible Host

B   Mode of Transmission

C    Infectious Agent

D    Reservoir

Answer:  Infectious Agent

Link in the Chain of Infection Description


Infectious Agent ·        The pathogen causing the infection e.g., bacteria, viruses, fungi, parasites.

·        Characteristics of pathogen include virulence, resistance and the ability to survive and multiply.




Reservoir ·        The source or host where the infectious agent resides and reproduces.

·        Can be human, animal, or environmental (e.g., contaminated water or soil).

·        Reservoirs may be symptomatic or asymptomatic carriers.



Portal of Exit ·        The pathway through which the infectious agent leaves the reservoir.

·        For examples respiratory secretions, blood, urine, feces and open wounds.



Mode of Transmission ·        The method by which the infectious agent is transferred from reservoir to a susceptible host.

·        Modes include direct contact and indirect through contaminated objects.



Portal of Entry ·        The route through which the infectious agent enters a susceptible host.

·        Includes mucous membranes, respiratory, gastrointestinal and genitourinary tracts.




Susceptible Host ·        The individual who is at risk of becoming infected.

·        Factors affecting susceptibility include age, immunity, overall health and genetics.

·        The presence of other illnesses can increase susceptibility.