Yellow fever is an infection caused by an arbovirus (arthropod-borne virus -- a virus carried by insects). There are over 300 known arboviruses, over 150 of which are known to infect humans; other arboviral diseases include several types of equine encephalitis and other kinds of encephalitis including dengue fever, St. Louis encephalitis, and West Nile virus, The latter three diseases, and yellow fever, are caused by viruses belonging to the "Flaviviridae" family of arboviruses.
Most arboviruses are usually found in nature in birds and small animals, and people are infected accidentally when bitten by carrier insects, or when transfused with blood from an infected donor. Yellow fever and dengue fever are exceptions: these viruses only infect humans, and are carried from person to person by insects. Yellow fever has three stages. Stage I consists of fever, malaise (feeling lousy), headaches, weakness, nausea, and vomiting (sounds like the flu, doesn't it?). In stage II most of the symptoms go away, or at least get somewhat better. Stage III consists of fever, bleeding (anywhere, including the nose or the gums -- the combination of fever and bleeding is called hemorrhagic fever), liver failure with jaundice (thus the term "yellow fever"), kidney failure with very low urine output, low blood protein, and failure of other body systems including the circulatory system.
Like most viral diseases, there are no antibiotics available for yellow fever. Close clinical monitoring and supportive treatment (such as IV fluids to keep the patient hydrated, replacement of depleted blood proteins, and dialysis) may help, but one out of two people with yellow fever die of the disease.
Since yellow fever is transmitted from person to person by insects, travellers in the tropics should take care to avoid mosquitoes and other biting insects. Insect repellents are important, and clothing should cover most of the skin; you should sleep in screened and/or air-conditioned areas.
There is a vaccine available for yellow fever. It can be given safely to most people age 9 months to 50 years, and immunity lasts for at least 10 years and may even last for a lifetime. Children younger than 9 months may develop encephalitis after receiving the vaccine, and the risk is high enough before age 4 months that we do not give the vaccine to children that young. Side effects involving the digestive and nervous systems are occasionally seen in adults -- mainly in people older than 50, but sometimes in younger people as well. The vaccine is manufactured from chicken eggs, so people allergic to eggs should not receive the vaccine. Pregnant and breastfeeding women and people with immune-system problems should receive the vaccine only if their risk of being infected is very high; this depends a lot on where they will be travelling. Travellers to and from certain countries, mostly in Africa and South America, must receive the vaccine every 10 years in order to travel unless they have valid medical reasons not to be vaccinated, and those who cannot be vaccinated must carry papers saying why. The vaccine is available only in vaccination centers approved by public health authorities. For information on where one can be vaccinated in the United States, and on which countries require yellow fever vaccination for entry, see the CDC Web site.