Influenza (Flu) Influenza, commonly called the flu, is a contagious respiratory illness caused by influenza viruses. It can range from mild to severe and may lead to complications, especially in older adults, young children, pregnant people, and those with certain chronic medical conditions. The flu is different from a common cold. Flu symptoms often come on suddenly and can make you feel much more tired, achy, and feverish. Common flu symptoms Flu symptoms may include: Fever