Yellow fever is an infectious epidemic primate disease caused by a virus in the family flaviviridae. It is transmitted to (and between) humans by mosquitoes and is is characterized by severe headache, muscle aches, low blood pressure, fever, jaundice and severe hepatitis. The disease gets its name from the yellowing of the skin caused by the jaundice. The liver damage from this viral illness can be fatal. The virus causes an estimated 200,000 infections and 30,000 deaths worldwide each year.

The yellow fever virus is originally endemic to Africa; historical texts mention the illness from the 1600s on. The disease (and the mosquito that carried it, Aedes aegypti) originally came to the Carribean on slave ships from West Africa. From the original colonies, it spread throughout tropical areas of the New World. Mosquitos in the genus Haemogogus became its new vector in South America.

It caused a huge number of deaths during the construction on the Panama Canal in the early 1900s. An American army doctor named Walter Reed figured out that the disease was transmitted by mosquitos. Part of his discovery was based on observations in the 1860s by Havanna physician Carlos J. Finlay who believed that mosquitos spread the "yellow jack" fever that plagued many people on the islands. Dr. Reed demonstrated in the lab that there was a filterable agent in fever patient's blood that caused the infection. As a result of his and later researcher's work, the yellow fever virus was the first human pathogen to be isolated and identified as a virus.

After infection, it takes from three to six days before a person starts suffering symptoms. Generally, a victim experiences acute symptoms first: high fever, muscle pain (often with a bad backache), headache, chills, loss of appetite, nausea, and vomiting. A hallmark of the disease is that along with the fever, the patient's heartbeat slows way down and his or her blood pressure often drops. A few lucky people experience no acute symptoms, but will act as carriers of the disease. In any case, the acute symptoms gradually lessen and go away after three or four days.

However, 24 hours after the subsidence of the acute phase, about 15% of infected people will enter a so-called "toxic phase". In this phase, the fever reappears along with inflammation and bleeding in internal organs. The patient appears jaundiced, and will usually be experiencing severe stomach pains along with vomiting. He or she may also be hemorrhaging from the mouth, nose and eyes, and blood will be in the urine and feces. About half of the patients who enter this phase will die within 10-14 days of severe liver damage or kidney failure. The rest will gradually recover.

Diagnosing yellow fever can be tricky in its early stages, since it can look a lot like related diseases such as dengue, malaria, typhoid, or even food poisoning. The disease can be accurately identified with a blood test to check for characteristic yellow fever antibodies.

There is no good treatment for the infection once it's begun. Medical assistance is generally supportive to treat the symptoms as they appear. Health care practitioners will administer fluids to prevent dehydration, medicines to treat the pain and fever, etc.

The best way to prevent infection with yellow fever is by getting vaccinated. A highly effective vaccine was developed in the 1940s by a researcher named Max Theiler. The modern version of this vaccine uses live attenuated yellow fever virus and produces mild side effects in only five percent of the people who receive it.

People who travel to or live in afflicted tropical areas must get vaccinated every ten years, but for most people, a single vaccination confers lifelong immunity to the disease. The World Health Organization has been trying to get yellow fever vaccinations incorporated as a routine part local childhood vaccination programs in afflicted areas of South America and Africa, but progress on this has been slow in some areas.