Influenza, commonly known as the flu, is a highly contagious respiratory illness caused by the influenza virus. It spreads easily through coughing, sneezing, and contact with contaminated surfaces. Symptoms include fever, cough, sore throat, body aches, fatigue, and in severe cases, pneumonia. Prevention measures include getting a flu vaccine, washing hands frequently, and avoiding close contact with infected individuals. Treatment includes rest, fluids, and antiviral medications. In some severe cases, hospitalization may be necessary.