Influenza, commonly called the flu, is a contagious respiratory illness. It is caused by viruses that infect the nose, throat and sometimes the lungs. It often begins suddenly with fever, cough, sore throat, body aches and fatigue. Symptoms can range from mild discomfort to severe illness.
There are four types of influenza viruses: A, B, C and D. Only types A and B regularly cause seasonal outbreaks in people. Influenza spreads easily through droplets when someone coughs, sneezes or talks. Most healthy individuals recover on their own in about a week, but the flu can lead to serious complications for others.