Influenza, commonly known as the flu, is an infectious disease caused by rapidly evolving influenza viruses, primarily types A and B, which are responsible for seasonal epidemics in humans. Symptoms typically include fever, cough, muscle pain, and fatigue, ranging from mild to severe and potentially leading to serious complications like pneumonia. Transmission occurs mainly through respiratory droplets from coughing and sneezing, with prevention strategies emphasizing frequent hand washing, mask-wearing, and annual vaccination updated to match circulating strains.
Globally, influenza affects 5-15% of the population annually, resulting in 3-5 million severe cases and up to 650,000 respiratory-related deaths, disproportionately impacting high-risk groups such as young children, the elderly, and individuals with chronic health conditions. While often self-limiting in healthy people, severe infections can be treated with antiviral drugs. Historically, novel influenza strains have caused devastating pandemics every 10-50 years, including the most severe Spanish Flu (1918-1920) and the Swine Flu pandemic in 2009.