Yellow fever virus Technical Description: Yellow Fever Close Window The causative agent in yellow fever, this virus remains a potential biological warfare (BW) threat. Long considered a potential BW agent, the United States was especially concerned about yellow fever during World War II. This interest was piqued when Japanese scientists attempted to acquire samples of the virus from the Rockefeller Institute in 1939. Since then, the United States and presumably other countries with BW programs have investigated the potential of yellow fever in biological weaponry, if only for defensive purposes. (Most of the literature with regard to BW agents generally classifies yellow fever under the rubric of hemorrhagic fevers, without much specific data regarding its past weaponization.) Classified as a flavivirus, yellow fever is transmitted by insects and is still a global health problem, but was particularly devastating in the southern hemisphere (Africa and South America) before mosquito abatement and vaccines were introduced. Two epidemiological cycles exist in nature. The first, jungle yellow fever, is a disease of monkeys, with arthropod-borne transmission by mosquitoes of the Haemogogus species, in which humans are accidental hosts. The second, urban yellow fever, is transmitted by the Aedes mosquito, and humans are the primary reservoir. Following a three- to six-day incubation period, yellow fever is often characterized by two relatively distinct phases of disease. First, backache, chills, muscle pain, with nausea and sometimes vomiting. Most cases resolve following this stage in about three-four days. Some (approximately 15 percent) of those infected with yellow fever go on to develop a “toxic phase” of the disease, and half of these usually do not survive. Symptoms in these cases are severe, with abdominal pain, vomiting, and bleeding from the mouth, eyes, and stomach. Kidney failure can occur resulting in death within two weeks. Overall, the fatality rate in yellow fever is estimated at 5-10 percent. While no antiviral therapy exists, a live, attenuated vaccine, with protection lasting up to 10 years, is currently available for travelers to and residents within endemic areas.