Influenza, or the Flu, is a contagious respiratory illness caused by influenza viruses. It attacks the lungs, nose, and throat. Symptoms include fever, chills, muscle aches, cough, congestion, runny nose, headaches, and fatigue. It is a seasonal infection that typically occurs during the fall and winter. The Flu can affect people of all ages and lead to a wide range of symptoms, from mild to severe. Young children, older adults, pregnant women, and people with chronic diseases or weak immune systems are at high risk.