Interval between primary and secondary cases ?

Ans. is 'c' i.e., Serial interval Latent period, serial interval and generation time, all are approximate measure of incubation period, i.e., these period can give some idea about incubation period. Latent period The period from disease initiation to disease detection. It is used in non-infectious disease as the equivalent of incubation period in infectious disease. Serial interval The gap time between onset of primary case and secondary case. By collecting information about a whole series of such onset, we get a distribution of secondary cases from which we can guess the incubation period. Generation time Interval between receipt of infecton by host and maximal infectivity of the host. Generation time is roughly equal to the incubation period. About option 'b' Communicable period The time during which an infectious agent may be transferred directly or indirectly from an infected person to another person. The period of communicability has no relation with incubation period, it merely reflects the duration when the infectious agent may be transferred. This may occur during incubation period, during actual illness or during convalsescence. Generally communicable diseaes are not communicable in incubation period except - measles, Chicken pox, Peussis hepatitis A, i.e., these diseases are communicable during their late incubation period.