The Rift Valley virus, common in Africa, was first isolated in 1931 among livestock on a farm in Kenya. The virus usually lies dormant in the dried eggs of the Aedes mosquito. During floods and heavy rains, these eggs hatch infected mosquitoes. The Aedes mosquito normally transmits the virus by biting goats, camels and cattle. The disease, one of a family known as phleboviruses, can then be transmitted to humans by sandflies and mosquitoes who bite the infected animals.
People can also catch the illness when they slaughter or eat an animal suffering from the fever.
In Egypt a severe epidemic of Rift Valley Fever killed 600 people in 1977 and 1978. In humans the death rate averages 3 to 4 percent. Those who die are usually malnourished, sick with other diseases or far from good medical care. About half the infected people have no symptoms. In northeastern Kenya and western Somalia, recent floods have left many people in a very weakened condition.