Rift Valley fever (RVF) is a viral zoonosis (affects primarily domestic livestock, but can be passed to humans) causing fever. It is spread by the bite of infected mosquitoes. The disease is caused by the RVF virus, a member of the genus Phlebovirus (family Bunyaviridae). The disease was first reported among livestock in Kenya around 1915, but the virus was not isolated until 1931. RVF outbreaks occur across sub-Saharan Africa, with outbreaks occurring elsewhere infrequntly (but sometimes severely - in Egypt in 1977-78, several million people were infected and thousands died during a violent epidemic; in September 2000 an outbreak was confirmed in Saudi Arabia and Yemen).

In humans the virus can cause several different syndromes. Usually sufferers have either no symptoms or only a mild illness with fever, headache, myalgia and liver abnormalities. In a small percentage of cases (< 2%) the illness can progress to hemorrhagic fever syndrome, meningo-encephalitis (inflammation of the brain), or affecting the eye. Patients who become ill usually experience fever, generalized weakness, back pain, dizziness, and weight loss at the onset of the illness. Typically, patients recover within 2-7 days after onset.

Approximately 1% of human sufferers die of the disease. Amongst livestock the fatality level is significantly higher. In pregnant livestock infected with RVF there is the abortion of virtually 100% of fetuses. An epizootic (animal disease epidemic) of RVF is usually first indicated by a wave of unexplained abortions.