Influenza or flu, acute, highly contagious disease caused by any one of at least three types of virus, but usually by types designated A and B. Formerly known as the grippe, influenza is difficult to diagnose in the absence of an epidemic, since it resembles many common respiratory ailments. It can be distinguished from a cold, however, by sudden fever, prostration, weakness, and sometimes severe muscular aches and pains. It is spread by respiratory droplets.