Influenza A is a type of virus that causes influenza, a highly contagious respiratory illness. Influenza A is one of the four types of influenza viruses, the others being influenza B, C, and D). Influenza A and B viruses cause seasonal epidemics of disease in people, known as flu season, almost every winter. Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively)). Currently circulating in humans are subtype A(H1N1) and A(H3N2) influenza viruses). Most people who have the flu are infected with the influenza type A virus. Symptoms of influenza A infection include fever, chills, muscle pains, coughing, and a sore throat. Influenza A infection can be diagnosed through a PCR test, which can tell influenza A apart from influenza B and other similar viruses. Influenza A infection can be prevented by vaccination, which is recommended for all individuals over 6 months old, especially those at high risk.