How is immunity defined in the context of infectious diseases?

• A. The ability to mount fever after infection
• B. The probability of contracting disease upon exposure
• C. The capacity of a person when exposed to an infectious agent to remain free of infection or clinical illness
• D. The requirement to vaccinate to avoid illness

Answer: (C)

Immunity is the body's ability to resist infection or disease when exposed to an infectious agent. When someone is immune, exposure may not lead to infection at all, or it may prevent clinical illness even if infection occurs—this includes scenarios where the immune system stops the pathogen or quickly controls it, reducing or eliminating symptoms. This can happen with natural immunity from prior exposure or infection, or vaccine-induced immunity that trains the immune system to respond effectively.

Fever is a sign of the body's response to infection, not a measure of immune status, so it’s not the definition of immunity. The chance of contracting disease upon exposure describes risk or attack rate, not the protective state itself. Vaccination helps establish immunity, but immunity is the protection you have, not the act of getting vaccinated.