What Is It?
Influenza (the flu) is a respiratory infection. It is caused by the influenza virus. Influenza typically is spread by air or by direct contact from one person to another. Influenza virus is very contagious.
Most influenza cases occur during epidemics. Epidemics peak during the winter months. A particularly widespread and severe epidemic is called a pandemic.
Compared with other viruses, influenza can strike remarkably large numbers of people in a relatively short time. In the developed nations, about 10-15% of the people get the flu each year. During severe epidemics, a greater fraction of the population gets sick.