H1N1 influenza, also known as swine flu, is a respiratory illness caused by the H1N1 virus. It was first identified in 2009 and can spread from person to person, similar to seasonal flu. Symptoms include fever, cough, sore throat, body aches, and fatigue. Most people recover without needing medical treatment, but some may develop severe complications. The H1N1 virus is part of the influenza family and can affect people of all ages. Vaccines are available to help prevent infection, and public health measures, such as handwashing and staying home when sick, can reduce its spread.