Incubation period is defined as -

Ans. is 'c' i.e., From receipt of infection to appearance of clinicial feature * Incubation period is the time interval between invasion by an infectious agent and appearnace of the first sign and symptom. 'Median incubation period' is the time required for 50% cases to occur following exposure. Incubation period is useful for: (i) Tracing source of infection; (ii) Period of surveillance or quarantine; (iii) Immunization; (iv) Prognosis; and (v) Identification of point source epidemic. It has no relation with period of isolation.* Latent period is the period from disease initiation to disease detection.* Serial interval is the gap time between onset of primary and secondary case.* Generation time is the interval between receipt of infection by host and maximal infectivity of the host. Generation time is roughly equal to incubation period.* Communicable period is time during which an infectious agent can be transferred from infected person to others. It has no relation with incubation period.