The term COMMON VIRAL INFECTION can be summarized by the three-letter solution FLU. Influenza, commonly known as the FLU, is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe symptoms, and in some cases can even lead to death. Symptoms of the FLU include fever, chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and fatigue. The FLU is spread through contact with an infected person's respiratory droplets, which are released when they cough, sneeze, or talk.