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classification. Staging can aid in defining uniform study groups for research studies, determining
treatment regimens, predicting prognosis, and in providing intermediate end-points for clinical trials.
Natural history therefore includes a presymptomatic period and a postmorbid period. Of
particular interest for epidemiologists is the former, the period of time before clinical manifestations
of the disease occur and, for infectious diseases, the period of time between infection and
infectiousness. For non-infectious diseases, the term induction period refers to the "period of time
from causal action until disease initiation" (Rothman and Greenland, p14). The induction period
may be followed by a latent period (also called latency), which is the "time interval between disease
occurrence and detection" (Rothman and Greenland, p15). This distinction, though not made by all
authors, is important for diseases that can be detected through screening tests, since the latent
period represents the stage of the disease natural history when early detection is possible.
The distinction is also important for designing epidemiologic studies. Since the time of disease
detection may be advanced through the application of screening and diagnostic tests, the number of
cases detected can change with technology. Also, the collection of historical exposure data should
be guided by a concept of when such exposure would have been biologically relevant. For a factor
believed to contribute to the initiation of a disease, exposure must occur before that point. For a
factor believed to contribute to promotion or progression of the condition, exposure can take place
following initiation.
For infectious diseases, there are two markers of epidemiologic importance: disease detection and
the onset of infectiousness. Incubation period refers to the "time from infection to development
of symptomatic disease" (Halloran, p530). This term is sometimes applied to non-infectious
diseases, but often without a precise meaning. The incubation period thus covers both the
induction and latent periods as these are defined for non-infectious diseases. In contrast, the term
latent period has a different meaning for infectious diseases, where it denotes "the time interval from
infection to development of infectiousness" (Halloran, p530). Since an infected person may be
infectious before developing symptoms, while symptomatic, or after disappearance of symptoms,
there is no absolute relationship of incubation and latent periods for infectious disease. Relevant
concepts are inapparent or silent infection (asymptomatic, either infectious or non-infectious) and
carrier (post-symptomatic but still infectious) (Halloran, pp530-531).
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rev. 9/10/2000, 5/8/2001