The interval between successive cases of an infectious disease.

The interval between successive cases of an infectious disease is determined by the time from infection to infectiousness, the duration of infectiousness, the time from infection to disease onset (incubation period), the duration of any extra-human phase of the infectious agent, and the proportion clinically affected among infected individuals. The interval is important in the interpretation of infectious disease surveillance and trend data, in the identification of outbreaks, and in the optimization of quarantine and contact tracing. This paper discusses the properties of these intervals, as measured between transmission events or between clinical onsets of successive infected individuals, noting the determinants of their ranges and frequency distributions, the circumstances under which secondary cases may arise before primaries, and under which the infection transmission interval will be different from the interval between clinical onsets of successive cases. It discusses the derivation of interval distribution statistics from descriptive data given in standard textbooks, with illustrations from published data on outbreaks, households, and epidemiologic tracing. Finally, it discusses the implications of such measures for studies of secondary attack rates, for the persistence of infection in human communities, for outbreak response, and for elimination or eradication programs.