An arthropod-borne (primarily mosquito), acute, febrile, viral disease of humans and numerous species of animals, particularly ruminants. Rift Valley fever is caused by a ribonucleic acid (RNA) virus in the genus Phlebovirus of the family Bunyaviridae. In sheep and cattle, it is also known as infectious enzootic hepatitis. First described in the Rift Valley of Africa, the disease presently occurs in west, east, and south Africa and has extended as far north as Egypt. Outside of Africa, Rift Valley fever has spread to the island of Madagascar and the Arabian Peninsula (Saudi Arabia and Yemen).