Influenza, also known as the flu, is a contagious disease that is caused by the influenza virus. It attacks the respiratory tract in humans (nose, throat, and lungs). The flu is different from a cold. Influenza usually comes on suddenly and may include these symptoms: fever, headache, tiredness (can be extreme), dry cough, sore throat, nasal congestion and body aches. These symptoms are usually referred to as "flu-like symptoms."

Influenza A viruses periodically cause worldwide epidemics, or pandemics, with high rates of illness and death. Unlike other public welfare emergencies, an influenza pandemic will impact on multiple communities across the United States and require swift and coordinated action and cooperation by all levels of government. Advanced planning for a large scale and widespread health emergency is required to optimize health care delivery during a pandemic. In addition, prevention and preparedness activities facilitate the response and recovery during and after an influenza pandemic.