Merck Manual

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Yellow Fever


Thomas M. Yuill

, PhD, University of Wisconsin-Madison

Last full review/revision May 2020| Content last modified May 2020
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Yellow fever is a mosquito-borne viral disease that occurs mainly in the tropics.

  • Yellow fever occurs only in the tropical areas of Central Africa, southern Panama, and South America.

  • Some people with yellow fever have no or mild symptoms, but others have more severe symptoms such as yellow skin (jaundice), fever, headache, muscle aches, and bleeding.

  • Doctors diagnose yellow fever by growing (culturing) the virus, by doing blood tests to detect antibodies to the virus, or by using polymerase chain reaction (PCR) techniques to help identify the virus's genetic material.

  • A vaccine is available for use by people residing in endemic countries and for travelers to areas where yellow fever is common, but avoiding mosquito bites is also important.

  • Treatment is mainly supportive and includes drugs to treat or prevent bleeding.

Yellow fever is caused by a flavivirus that is spread by mosquitoes.

Yellow fever is one of the most easily recognized and historically important viral infections. In the past, major epidemics of yellow fever caused tens of thousands of deaths. Once common in tropical and temperate zones around the world, the disease now occurs only in the tropical areas of Central Africa, southern Panama, and South America. Infection is more common during hot, rainy, humid months in South America and during the late rainy and early dry seasons in Africa.


Some infected people do not have symptoms. Others have mild symptoms, and some have a severe, life-threatening illness.

Symptoms of yellow fever usually appear about 3 to 6 days after being bitten by an infected mosquito. The first symptoms are headache, dizziness, muscle aches, chills, and mild fever, which begin suddenly. Nausea, vomiting, constipation, extreme fatigue, irritability, and restlessness are common. The face is flushed.

All of these symptoms subside after a few days. Some people then recover, but others develop a high fever, nausea, vomiting, and severe generalized pain a few hours or days after the initial symptoms subside. The skin turns yellow (jaundice) because the liver is infected. Often, there is bleeding from the nose, mouth, and gastrointestinal tract. People may vomit blood. They may become confused and apathetic.

Some people become delirious. They have very low blood pressure (shock). Severe infection can cause seizures, malfunction of several organs, and coma may occur. Up to 50% of people with severe bleeding and fever die.


  • Culture or blood tests

Doctors suspect yellow fever when people living in an area where the infection is common have typical symptoms.

Yellow fever is diagnosed by growing (culturing) the virus or detecting antibodies to the virus in the blood. Or polymerase chain reaction (PCR) techniques may be used to make many copies of the virus's genetic material. This technique enables doctors to rapidly and accurately identify the virus.


Prevention of yellow fever involves

  • Avoiding mosquito bites

  • Vaccination

  • Isolation

Avoiding mosquito bites is key to prevention. People who live in or visit areas where yellow fever is common can

  • Apply DEET (diethyltoluamide) insect repellant to the skin

  • Use mosquito netting

  • Wear long-sleeved shirts and long pants

A vaccine that is 95% effective at preventing yellow fever is available. People should be given the vaccine at least 10 days before traveling to a yellow fever endemic country. In the United States, the vaccine is given only at yellow fever vaccination clinics authorized by the U.S. Public Health Service. However, there are many such centers (see CDC: Yellow Fever Vaccination Clinics). Vaccination provides immunity for life.

Many countries require vaccination only for travelers coming into their country from areas where yellow fever occurs. If people are traveling to areas where yellow fever is common, they should be vaccinated.

The vaccine is not given to

  • Pregnant women

  • Infants under 6 months old

  • People with a weakened immune system, such as those with AIDS

If the infection is suspected or diagnosed, people are isolated in rooms that are screened and sprayed with insecticides to prevent further spread of the virus by mosquitoes.


  • Supportive care

Treatment of yellow fever involves supportive care, including drugs to treat or prevent bleeding, such as injections of vitamin K (which can help blood clot).

There is no specific treatment for yellow fever.

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