The flu is an infectious disease caused by the influenza virus. It affects the respiratory system and can cause symptoms such as fever, cough, sore throat, runny nose, muscle aches, and fatigue. Flu viruses are highly contagious and can spread easily through person-to-person contact or by inhaling infected droplets from the air. The flu can range in severity from mild to severe, and may require medical attention in some cases. Vaccination is an effective way to prevent the flu.