Influenza (or flu) is a highly contagious viral respiratory tract infection. An estimated 5 to 20 percent of people in the U.S. get influenza each year. Influenza is characterized by the abrupt onset of fever, muscle aches, sore throat, and a nonproductive cough.
Influenza can make people of any age ill. Although most people are ill with influenza for only a few days, some have a much more serious illness and may need to be hospitalized. Influenza can also lead to pneumonia and death.
Influenza viruses are divided into three types, designated as A, B, and C.
Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates for hospitalization and death. Efforts to control the impact of influenza are focused on types A and B.
Influenza type C usually causes either a very mild respiratory illness or no symptoms at all. It does not cause epidemics and does not have the severe public health impact that influenza types A and B do.
Influenza viruses continually mutate or change, which enables the virus to evade the immune system of its host. This makes people susceptible to influenza infection throughout their lives. The process works as follows:
A person infected with influenza virus develops antibody against that virus.
The virus mutates or changes.
The "older" antibody no longer recognizes the "newer" virus.
The older antibody can, however, provide partial protection against reinfection. Currently, three different influenza viruses circulate worldwide: two type A viruses and one type B virus. Immunizations given each year to protect against the flu contain the influenza virus strain from each type that is expected to cause the flu within that year.