The H1N1 influenza virus, commonly known as the swine flu, is a strain of the influenza virus that can infect humans. It was first identified in 2009 and is known for causing respiratory illness. The virus spreads through respiratory droplets when an infected person coughs or sneezes, and it can also be transmitted by touching contaminated surfaces. Symptoms of H1N1 include fever, cough, sore throat, body aches, and fatigue. Most people recover without needing medical treatment, but severe cases can lead to complications, especially in vulnerable populations such as the elderly, young children, and those with underlying health conditions. Vaccines are available to help prevent infection.