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Influenza, an acute contagious respiratory infection caused by orthomyxoviruses A, B, and C, occurs in local outbreaks, epidemics and pandemics. Differences in the morphology and antigen patterns for these viruses account for the differences in virulence and immunity. Influenza A is virulent, B is mild, and C is nonvirulent. Swine influenza virus may spread from swine to man (recycling of human A virus), causing epidemics and pandemics. The 1976-77 influenza vaccination program involved swine virus, and was associated with 321 cases of Guillain-Barré syndrome and a mortality of 5 percent. Subsequent yearly programs with other types of vaccine are described. Dosages vary according to the age of the vaccinee. Those who need vaccination include persons over 65 years of age, patients with heart/lung, liver/lung or metabolic diseases, severe anemia, or a compromised antigenic system. Side effects include local reaction, fever, an allergic reaction (due to egg protein), and the Guillain-Barré syndrome (only with swine virus vaccine). In general, influenza vaccine is safe and confers 60-85 percent protection. Since immunity lasts only about a year, annual vaccination with recently isolated virus vaccine is recommended. Amantadine seems effective therapeutically and prophylactically for clinical influenza A infection.