The term COMMON VIRUS can be summarized by the crossword solution FLU, which is a three-letter word. The FLU, or influenza, is a contagious respiratory illness caused by a virus. It is spread through coughing, sneezing, and contact with infected surfaces. Symptoms of the FLU include fever, chills, body aches, and fatigue. Vaccines are available to help prevent the FLU, and antiviral medications can be used to treat it.