Influenza A and B viruses exhibit frequent minor antigenic drift and type A viruses undergo a major antigenic shift every one to four decades, thus assuring that at least a portion of the population is always susceptible. Children and young adults have the highest incidence of influenza infection each winter, but the highest incidences of severe or complicated influenza illness leading to hospitalization or death are in infants, elderly persons (especially those in nursing homes), and persons of all ages with underlying heart or lung disease. Influenza viruses infect respiratory epithelial cells and can themselves cause diffuse pulmonary infiltrates and severe hypoxia, but concomitant or secondary bacterial pneumonia is a much more frequent complication of influenza. Although pneumococci predominate in these secondary pneumonias, the relative incidence of Staphylococcus aureus pneumonia also increases during influenza epidemics; empiric antibiotic therapy in this setting should be directed against both of these organisms. A variety of other bacteria can cause postinfluenzal pneumonia, especially in patients with alcoholism or chronic obstructive pulmonary disease, and broad antimicrobial coverage including gram-negative bacteria is justified in such patients when diagnostic studies provide no guidance. Early amantadine therapy of influenza-like illness during an influenza A epidemic will reduce the duration of symptoms and possibly reduce complications. Successful therapy of influenza virus pneumonia with ribavirin aerosol has been reported but not yet officially approved. Annual vaccination of persons at greatest risk for severe or complicated influenzal disease will reduce the morbidity and mortality due to this infection.(ABSTRACT TRUNCATED AT 250 WORDS)