Epidemiology of influenza.

The impact of influenza has been recognized for centuries. Its seasonality in temperate climates has allowed estimates of mortality and severe morbidity, such as hospitalization, to be made statistically, without identifying cases virologically. Most influenza related mortality occurs in older individuals and those with underlying conditions. In addition to those groups, influenza hospitalizations occur in younger children and pregnant women. Morbidity is more difficult to identify and laboratory confirmation is required for precise estimates to be made. Younger individuals experience the highest frequency of illnesses caused by all subtypes. This has resulted in suggested strategies for community control by vaccinating children.