An estimated 10 to 20 percent of the population in the US contract 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.
A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates of 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. 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.