Influenza type A is a type of virus that causes the flu, a highly contagious respiratory illness. It is one of the four types of influenza viruses, the others being types B, C, and D. 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 influenza A(H1N1) viruses are related to the pandemic 2009 H1N1 virus that emerged in the spring of 2009 and caused a flu pandemic. These viruses, scientifically called the “A(H1N1)pdm09 virus,” and more generally called “2009 H1N1,” have continued to circulate seasonally since then and have undergone genetic changes and changes to their antigenic properties (i.e., the properties of the virus that affect immunity). Influenza A infection is more serious than the common cold and can cause major outbreaks and severe disease. 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 people over 6 months old, especially those at high risk.