Epidemics of influenza infection typically occur during the winter months in temperate regions, and were responsible for an average of approximately 36,000 deaths/year in the U.S. during 1990-1999. Influenza viruses can also cause pandemics, during which rates of illness and death from influenza-related complications can increase worldwide. The rates of influenza A infection are highest among children, but rates of serious illness and death are highest among persons greater than 65 years old, children aged less than two years, and persons of any age who have medical conditions that place them at increased risk for complications from influenza A infection. Although influenza vaccination remains the cornerstone for the control and treatment of influenza, there is a strong need for new antiviral medications as an adjunct to vaccine therapy.