Influenza (the flu)
What it is (background) Influenza — commonly called “the flu” — is an acute viral infection of the nose, throat and lungs caused mainly by influenza A and B viruses. It spreads easily from person to person and can cause anything from a mild illness to severe disease and death, especially during seasonal epidemics or pandemics. PMC +1 Symptoms Typical flu symptoms appear suddenly and often include: Fever or feeling feverish/chills, cough, sore throat Runny or stuffy nose, muscle or body aches, headache Fatigue (tiredness) and sometimes vomiting or diarrhea (more common in children). Symptoms usually last about 5–7 days in healthy adults, though cough and fatigue can linger longer. If you’re in a high-risk group (older adults, very young children, pregnant people, or those with chronic medical conditions), symptoms can progress to severe complications like pneumonia. Mayo Clinic +1 Causes & how it spreads Influenza viruses mutate frequently, which is why new ...