You may need it to go from one country to another - especially if that country has that illness present. Yellow fever is a viral illness spread by mosquitoes in parts of Africa and South America. The vaccine is given by injection and lasts for approximately ten years. Minor reactions like pain at the injection site are the norm. This is an infection of the bloodstream by a parasite that is transmitted by the Anopheles mosquito after it bites you.
Hydration is very important.
Anyone coming back from the tropics with a fever should assume it is malaria until proven otherwise. It is a serious threat to travelers in the tropics. The best way to prevent malaria is to keep the mosquito from biting you. Avoid having skin that is exposed during the peak mosquito times - dawn and dusk. The Anopheles mosquito feeds at night.