Influenza epidemics account for more than 20,000 deaths in the United States each year, as well as substantial morbidity, medical costs, and time away from work and school. Since the 1950s, the principal weapon against these seasonal epidemics has been killed virus vaccine formulations. Despite massive efforts to immunize at-risk individuals against influenza, not everyone receives the vaccine. In addition, use of some drugs, such as amantadine and rimantadine, can lead to the development of drug resistant viruses in infected individuals and to transmission of these viruses to susceptible individuals. The many factors that contribute to the high annual incidence of influenza virus infections mandate prompt clinical recognition and appropriate patient management. Rapid diagnostic tests have been developed that may make it possible to avoid the use of antibacterial drugs, quickly decide whether isolation of infected patients is needed, and discharge hospitalized patients sooner.