yellow fever, acute infectious disease endemic in tropical Africa and many areas of South America. Epidemics have extended into subtropical and temperate regions during warm seasons. In 1878 a severe outbreak in the Mississippi Valley killed about 20,000; the last epidemic in the United States occurred in New Orleans in 1905. Yellow fever is caused by a virus transmitted by the bite of the female Aedes aegypti mosquito, which breeds in stagnant water near human habitations. A form of the disease called sylvan yellow fever is transmitted in tropical jungles by other species of mosquitoes that live in trees.

At the end of the 19th cent., yellow fever was highly prevalent in the Caribbean, and a way of controlling it had to be found before construction of the Panama Canal could be undertaken. In 1900 an American commission headed by Walter Reed and including James Carroll, Jesse Lazear, and Aristides Agramonte gathered in the U.S. Army's Camp Columbia in Cuba. Through their experiments they proved the theory of C. J. Finlay that yellow fever was a mosquito-borne infection. Within the next few years, W. C. Gorgas, an army physician and sanitation expert, succeeded in controlling the disease in the Panama Canal Zone and other areas in that part of the world by mosquito-eradication measures. The later development of an immunizing vaccine and strict quarantine measures against ships, planes, and passengers coming from known or suspected yellow-fever areas further aided control of the disease.

Yellow fever begins suddenly after an incubation period of three to five days. In mild cases only fever and headache may be present. The severe form of the disease commences with fever, chills, bleeding into the skin, rapid heartbeat, headache, back pains, and extreme prostration. Nausea, vomiting, and constipation are common. Jaundice usually appears on the second or third day. After the third day the symptoms recede, only to return with increased severity in the final stage, during which there is a marked tendency to hemorrhage internally; the characteristic "coffee ground" vomitus contains blood. The patient then lapses into delirium and coma, often followed by death. During epidemics the fatality rate was often as high as 85%. Although the disease still occurs, it is usually confined to sporadic outbreaks.

Acute infectious tropical disease, sometimes occurring in temperate zones. Abrupt onset of headache, backache, fever, nausea, and vomiting is followed by either recovery with immunity or by higher fever, slow pulse, and vomiting of blood. Patients may die in a week. Jaundice is common (hence the name). One of the world's great plagues for 300 years, it is caused by a virus transmitted by several species of mosquitoes. Carlos Finlay suggested and Walter Reed proved this means of spread, leading to near elimination of the disease through mosquito control (see William Gorgas). Treatment consists of supportive care, particularly fever reduction. Control of mosquitoes near cities and live-virus vaccines—developed by Max Theiler (1899–1972), who won a 1951 Nobel Prize for his work—have made yellow fever completely preventable.

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