Yellow fever is a disease caused by a virus transmitted by the mosquito Aedes. Its symptoms include fever, jaundice, bloody vomit, and it can cause death. Be sure to bring and use mosquito repellent if you are visiting a country where yellow fever is present.
A vaccine is available which provides immunity for at least ten years. Many countries will require that you be vaccinated against yellow fever if you are coming from a country where it is present or have been in such a country within the last 6 days (the incubation period). If this is the case you will need to present a "World Health Organisation International Certificate of Vaccination" with an appropriately dated certification on the "International Certificate of Vaccination or Revaccination Against Yellow Fever" page to immigration officials. This little yellow booklet will be given to you when you are vaccinated against yellow fever for the first time.
It is generally easiest to get a yellow fever vaccination in your home country before you leave on your trip. The vaccination is not in the normal vaccination schedules of many countries outside the infected areas since it is unnecessary unless you are travelling and has a somewhat higher risk of complications than many other vaccines. You will need to especially request a yellow fever vaccination from a designated yellow fever vaccination center. Immunity is considered to start 10 days from the time of the vaccination, so be sure to get vaccinated 10 days before you enter an infected country.
Note that in many cases you will need to allow about half an hour for a yellow fever vaccination as you will be observed for an allergic reaction to the vaccine after it is administered.
The following countries require or recommend immunization against yellow fever:
Brazil requires visitors from infected to be immunized. Yellow fever is endemic in Maranhão, Tocantins, Goiás, Mato Grosso do Sul, parts of Minas Gerais, São Paulo, and Paraná, and all states northwest.