Which term describes the ability to enter, survive, and multiply within a host?

• A. Infectivity
• B. Pathogenicity
• C. Virulence
• D. Immunogenicity

Answer: (A)

The main concept here is infectivity—the ability of a pathogen to establish infection in a host by entering, surviving, and multiplying. This term specifically captures the early steps of infection: getting into the body, persisting, and reproducing enough to be detectable as an infection.

Pathogenicity, by contrast, refers to the capacity to cause disease once infection has occurred—essentially, among those infected, how many actually develop illness. Virulence describes the severity of disease once it occurs, such as how serious or deadly it is. Immunogenicity is about the ability to provoke an immune response.

So, describing the capacity to enter, survive, and multiply within a host aligns with infectivity—the process of establishing infection itself—rather than the outcome (disease) or the strength of the disease or the immune response.