Dengue is an acute influenza-like disease caused by any of 4 genetically similar mosquito-borne arboviruses of the Flavivirus family. Dengue viruses are thus related to yellow fever virus, West Nile virus, Japanese encephalitis virus, and several other human pathogens. Although flaviviruses have historically been linked to specific geographic locales, in recent years many have been extending their geographic ranges. Among the most important examples of this phenomenon, dengue has been re-emerging globally, along with a potentially fatal, and poorly understood complication known as dengue hemorrhagic fever.