Influenza is a very contagious respiratory illness caused by influenza viruses. These viruses are spread when a person who has influenza coughs, sneezes, or speaks, releasing viruses into the air. When these viruses enter the nose, throat, or lungs of another person, they begin to multiply, causing disease. The viruses can also be spread when a person touches a surface with flu viruses on it (for example, a door handle) and then touches his or her nose or mouth.
Flu symptoms often include:
Flu can cause mild to severe illness, and at times can lead to death. Some people, such as older people, young children, and people with certain health conditions, are at high risk for serious flu complications. The best way to prevent the flu is by getting vaccinated each year.