Yellow fever is another of the vectored zoonotic diseases that take both mild and severe forms. It is caused by a Flavovirus of the Togaviridae family and is transmitted through the bite of several species of mosquitoes. For howler monkeys and spider monkeys, yellow fever is usually fatal. Capuchins, marmosets, and owl and squirrel monkeys are more resistant. Monkeys in the Americas are more susceptible than those in Africa, where the disease originated -- the only continents where the disease is found.
Human infection in regions where the disease is endemic, as well as "imported" cases found elsewhere in travelers, result from failure to be vaccinated or exposure to the virus in jungle settings too soon after vaccination. While the fatality rate in indigenous peoples is less than 5%, more severe cases in non-indigenous humans (cases developing liver and kidney failure and hemorrhage) is much higher, 20-50%. Control of yellow fever is a concerted international effort, and the prospect of global warming in conjunction with the irresponsibility of travelers may be the greatest threat to its success.