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.
The most common types of influenza virus are A and B. Influenza A is the one usually responsible for the annual epidemics. Most people get multiple flu infections during their lives. With many other types of infections, having the disease once protects against a second infection. That is because the body's immune system remembers the returning virus. It attacks it immediately, and rapidly eliminates it.
With influenza, the virus usually has mutated (changed) somewhat since the first infection. The change is enough to fool your immune system. As a result, the immune system responds slowly. By the time the immune response is in full gear, millions of the body's cells are already infected.