What is defined as the time from infection to infection of secondary cases?

• A. Incubation period
• B. Latent period
• C. Reproduction number
• D. Generation time

Answer: (D)

Generation time is the time between when a person becomes infected and the moment their infection leads to new infections in others. It measures how quickly transmission passes from one generation of cases to the next, not how long until symptoms appear or when a person becomes infectious.

This differs from the incubation period, which is from infection to the onset of symptoms, and from the latent period, which is from infection to when a person becomes infectious. It’s also distinct from the reproduction number, which is a count or rate (the average number of people one case will infect) rather than a time interval. While the serial interval (time between symptom onsets in successive cases) is related, generation time specifically tracks the infection-to-infection interval across generations.

So, the time from infection in the primary case to infection in the secondary cases it causes is generation time.