Seasonal influenza epidemics occur every year in the United States in varying degrees of severity. Between 1990 and 1999, flu epidemics caused approximately 36,000 deaths annually, primarily among the very young and the very old. Recently, heightened awareness triggered by reports of avian flu virus transmission to people in Asia has set plans in motion to respond to, and possibly prevent, an influenza pandemic that could cause illness and even death among millions in the United States and worldwide.