Dengue fever: An acute mosquito-borne viral illness of sudden onset with headache, fever, prostration, severe joint and muscle pain, swollen glands (lymphadenopathy), and rash. The presence of fever, rash, and headache (the 'dengue triad') is characteristic. Dengue fever is endemic throughout the tropics and subtropics. Also called breakbone fever, dandy fever, and dengue. Victims of dengue fever often suffer temporary contortions due to the intense joint and muscle pain.