What are the six parts of the chain of infection?

• A. Pathogen, Reservoir, Portal of Exit, Mode of Transmission, Portal of Entry, Susceptible Host
• B. Pathogen, Reservoir, Portal of Exit, Portal of Entry, Infectious Dose, Susceptible Host
• C. Pathogen, Reservoir, Portals of Exit, Mode of Transmission, Portal of Entry, Susceptible Host
• D. Pathogen, Reservoir, Portal of Exit, Mode of Transmission, Portal of Entry, Infected Host

Answer: (A)

The chain of infection is built from six links that must all be present for transmission to occur: the pathogen, the reservoir where it lives, the portal of exit from that reservoir, the mode of transmission by which it moves, the portal of entry into a new host, and a susceptible host who can be infected. This sequence matters because each step offers a point where intervention can break the chain and stop spread.

The listed correct sequence matches this standard model exactly: Pathogen, Reservoir, Portal of Exit, Mode of Transmission, Portal of Entry, Susceptible Host. Infectious dose is a related concept about how many organisms it takes to establish infection, but it is not one of the six links in the chain. Using “Portals” in plural and naming the host as an “infected host” misaligns with the established six-link framework, since the chain specifically includes a susceptible, not yet infected, host.