Flu is short for influenza, which is an infection of the nose, throat, and lungs caused by a virus. It is a highly contagious respiratory illness that spreads easily from person to person through the air or when people touch contaminated surfaces. Flu symptoms usually come on suddenly and can include fever, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, fatigue, and sometimes vomiting and diarrhea. While most people with the flu get better on their own, influenza and its complications can be deadly, especially for young children, older adults, and people with certain medical conditions. The best way to prevent the flu is by getting a flu vaccine each year.