Influenza viruses are unique in their ability to cause recurrent epidemics and truly global pandemics during which acute febrile respiratory disease occurs explosively in all age groups. Epidemics of varying severity occur almost annually in temperate climates and are punctuated by the much less frequent but more dramatic occurrence of pandemic influenza. Increases in hospitalization and death often accompany widespread morbidity during influenza epidemics and pandemics. Influenza pandemics also threaten to disrupt other essential and nonessential services through high absenteeism, with high economic losses resulting. The medical impact and disruptive effects of epidemics and pandemics clearly justify careful global monitoring of influenza and strenuous efforts to prevent this emerging and reemerging disease.