Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and sometimes can lead to complications like pneumonia, hospitalization, or death, especially in vulnerable populations such as young children, elderly adults, and individuals with underlying health conditions. Symptoms typically include fever, cough, sore throat, body aches, fatigue, and sometimes diarrhea or vomiting. The flu spreads primarily through respiratory droplets when an infected person coughs or sneezes. It can also be contracted by touching surfaces contaminated with the virus and then touching the face. Influenza is seasonal, typically peaking during the colder months. Vaccines are available and are recommended annually as the strains of the virus can change each year. Antiviral medications may be used to reduce the severity and duration of symptoms if administered early in the course of the illness.