The incubation period of influenza was estimated by utilizing Bailey's model from the results of an epidemiological survey concerned with the interval of incidence in the intra household of 1, 022 families of three persons in Osaka City in the epidemic of B type influenza in January and February 1961.
The following results were obtained:
1) The median and mode of distribution of the time interval between initial and continued cases of incidence was 6 days and 5 days respectively.
2) The incubation period of influenza was 7 days and the period of effective infection was 2 days.