infectious disease, caused by a virus, and characterized in severe cases by high fever, jaundice, hemorrhage, and kidney damage. Originally yellow fever was believed to be exclusively a disease of humans, but research revealed that it also affects monkeys and other animals. In the type of the disease known as jungle yellow fever, which occurs only sporadically in human beings, diseased monkeys of Africa and tropical America are the primary source of infection, and various species of carrier mosquitoes occasionally transmit the infection to humans entering the forest. On the other hand, so-called urban yellow fever, affecting people in populated areas, is generally transmitted from human to human, without monkey involvement, via the mosquito species Aedes aegypti, which lives close to human habitations and usually acquires the virus from humans. There is also an intermediate type of yellow fever, seen commonly in recent decades in the humid and semi-humid savannah of Africa but not outside Africa, in which mosquitoes infect both monkeys and humans, leading to small outbreaks in separate villages; this type of the disease is less often fatal.

Although a vaccine developed in the early 20th century provides a safe defense against yellow fever and has greatly reduced its scope. the incidence of yellow fever has increased somewhat in recent decades because of deforestation and urbanization, and large numbers of people in Africa and Asia remain unvaccinated. As of the end of the 20th century, according to the World Health Organization, some 200,000 cases of yellow fever were estimated to be occurring around the world each year, with 30,000 deaths.

History.

It is probable that yellow fever initially appeared in Africa and was brought to America at the time of the slave trade. The disease was first described in medical terms during the 17th century, when an outbreak was observed in the Yucatán Peninsula, Mexico. Ultimately yellow fever spread to the U.S., Spain, and other countries.

In the first half of the 19th century several physicians suggested a possible connection between mosquitoes and yellow fever. In the 1880s and 1890s the Cuban physician Carlos Finlay elaborated on the hypothesis that yellow fever is transmitted by mosquito bites, carrying out extensive experimentation. Finlay's theory was verified in 1901 by the work of several investigators, notably the American bacteriologist Walter Reed, who also proved the agent to be a virus. The disease was brought under control by advanced methods of sanitation, including drainage of mosquito breeding grounds and quarantine of ships arriving from infected areas.

Symptoms.

The incubation period of yellow fever is three to six days. Some infected persons suffer no symptoms at all. In others, the onset of symptoms is sudden, typically including headache, backache, shivers, and fever. The first stage may also be characterized by nausea, vomiting, loss of appetite, and presence of albumin in the urine. Most people recover after this stage. In mild cases the symptoms may disappear in one to four days. In more serious cases, after the initial fever, the temperature usually returns to normal, but soon the temperature again rises and a “toxic phase” begins. This stage is marked by jaundice, hemorrhage from mucous membranes, vomiting of blood (the characteristic, so-called black vomit of yellow fever), and degeneration of the liver, kidneys, and heart. The destruction of liver cells results in the accumulation of yellow bile pigments in the skin, giving the disease its name. As many as half of the patients in this phase die within ten to 14 days. In cases of spontaneous recovery, convalescence is usually rapid, although jaundice may persist for some time. The disease never recurs, one attack providing immunity for life.

Treatment and Prevention.

No treatment is known for yellow fever, but treatment is provided for symptoms. Intravenous fluids and blood products may need to be given to compensate for blood loss and for fluid and electrolyte imbalances. Dialysis may be required in the event of kidney failure.

Preventive measures include the avoidance of mosquito bites through the use of insect repellent, protective clothing, and mosquito netting. In 1939 the South African physician Max Theiler developed a vaccine that confers immunity to the disease. Vaccination is recommended for persons living in or traveling to traveling to endemic regions and is required by some countries. A single dose of vaccine confers immunity lasting 10 years or more.  Adverse reactions to the vaccine have been reported in recent years, but they are extremely rare.       T.H.W., THOMAS H. WELLER, M.S., M.D. & D.S.T., DOUGLASS S. THOMPSON, M.D.

For further information on this topic, see the Bibliography, sections 448. Viruses, bacteria, 508. Diseases.