Influenza, often called the flu, is an acute, highly contagious infection of the respiratory tract which commonly occurs in the winter.

Influenza usually appears in epidemic form and affects many people at once. Although it affects all age groups, the highest incidence occurs in school children. The greatest severity is in young children, elderly people, immunosuppressed people, and those with chronic diseases.

The most common complication of influenza is pneumonia, which can be primary influenza