Yellow fever is an acute viral hemorrhagic disease and occurs via the bite of an infected mosquito. Nonhuman and human primates are the main reservoirs of the virus (flavivirus).
The virus is only found in Sub-Saharan Africa and South America. The risk of infection is highest in West Africa and in the Amazon basin. The peak of infections occurs in the rainy season and at the beginning of the dry season.
The first symptoms appear three to six days after infection. Common signs of the disease include fever, jaundice and bleeding. Yellow fever is fatal in approximately 30% of cases.
Yellow fever is transmitted by mosquitoes (primarily Aedes aegypty) which are active from dawn to dusk, especially early in the morning and the late afternoon.
There is no cure, but the disease can be prevented by getting a vaccination. The vaccine is very effective and a single injection gives lifelong protection. The protective effect begins ten days after vaccination.
Some countries where yellow fever is endemic made vaccinations compulsory to prevent the spread of the virus. By encouraging such action, the World Health Organisation seeks to prevent the spread of the disease in regions where yellow fever has not yet been reported but where mosquitoes that can transmit the virus are present. You will need to present a valid certificate of vaccination to enter any such country. If you are going somewhere where the yellow fever vaccination is required by law or is strongly recommended, you should get the vaccine at least ten days prior to travel. You can check which countries this applies to on the country list on this website.