Rift Valley fever (RVF) outbreaks affecting humans and livestock occur across sub-Saharan Africa, with outbreaks occurring elsewhere infrequently. Outbreaks usually correspond with the El Niño, associated with increased rainfall and flooding, causing increases in the mosquito population, which act as vector for the virus. The earliest documented outbreak was identified in livestock in Kenya in 1931, and RVF is believed to have been circulating in the country earlier in the 20th century. The first recorded human deaths occurred in 1974 in South Africa.