Generation time:> It is the interval of time between receipt of infection by a host and maximal infectivity of that host. In general generation time is largely equal to the incubation period.> Serial Interval: It is the gap in time between the onset of primary case and the secondary case. Serial Interval also helps us to guess the incubation period.> Latent Period: Is used in non-infectious diseases as the equivalent to incubation period in infectious disease regarding period of communicability. It is the time during which a disease is communicable i.e. can be transferred from one agent to another. "'As a rule infectious disease are not communicable during the incubation periods but there are certain exceptions as for example measles chickenpox whooping cough and hepatitis are communicable during the later part of incubation period" So Period of communicability is not a proxy of Incubation period.> The communicable period is defined as 'the time during which an infectious agent may be transferred directly or indirectly from an infected person to another person.'> The period of communicability bears absolutely no relation to the incubation period, and merely reflects the duration when the infectious agent may be transferred.> This may occur during the incubation period or during the actual illness.">