Influenza is a contagious respiratory illness caused by two main types of the flu virus: Types A and B. The influenza A and B viruses that routinely spread in people are responsible for seasonal flu epidemics each year. Both can cause mild to severe illness with symptoms including fever, sore throat, runny nose, muscle aches, headache, coughing and general fatigue. Serious outcomes of infection can result in hospitalization or death. Some people, such as elderly people, young children and people with certain health conditions are at a higher risk of serious complications. The best way to prevent the flu is by getting vaccinated annually.