Influenza, an acute, usually self-limited, febrile illness of global importance, appears virtually every year and infects the respiratory tract either sporadically, as a local outbreak, or as a widespread epidemic. The most severe outbreak known, the 1918 to 1919 influenza A pandemic, was responsible for an estimated 20 million deaths globally. In the United States, the influenza pandemics of 1957 and 1968 were associated with an attack rate of up to 50% and an estimated 100,000 deaths. Interpandemic influenza is responsible for considerable morbidity and mortality, which exceed that associated with the introduction of the pandemic strain. Influenza B infections resemble those due to influenza A, but are associated with fewer deaths. Features of influenza include headache, myalgia, malaise, anorexia, sore throat, nonproductive cough, sneezing, and nasal discharge; these symptoms are not pathognomic for influenza, and asymptomatic infection can occur. The pulmonary complications of influenza include pneumonia (viral and bacterial), croup, asthma, and bronchitis. Myocarditis and pericarditis are occasional cardiac complications. In addition to Reye's syndrome, a range of neurologic complications have been noted, including confusion, convulsions, psychosis, neuritis, Guillain-Barré syndrome, coma, transverse myelitis, and encephalomyelitis. Influenza has also been associated with the toxic shock syndrome, myositis, myoglobinuria, and renal failure. In view of its enormous human and economic toll, influenza remains a major target for improved vaccines and vaccine delivery, and antiviral treatment and prophylaxis.