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Although the specific diagnosis of influenza requires laboratory confirmation,
epidemics caused by influenza A have certain distinctive epidemiologic features
which may be helpful for making an etiologic diagnosis. In the Temperate Zone,
influenza occurs typically in late fall and winter. It causes high rates of
absenteeism in schools and industries. As the epidemic progresses, there is an
excess mortality manifested primarily in the elderly population and among those
with chronic conditions such as heart disease, chronic lung disease, diabetes,
and renal disease. The epidemic disseminates through the community over five or
six weeks.
High attack rates of acute respiratory illness among school children and high
mortality among the "aged and debilitated" are almost pathognomonic of
influenza A.
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