Travellers are advised to get a vaccination against hepatitis A, typhoid, yellow fever and polio. Those arriving from an infected country are required to hold a yellow fever vaccination certificate. There is a risk of malaria all year and outbreaks of Rift Valley Fever occur; travellers should take precautions to avoid mosquito bites. Sleeping sickness is a risk in the game parks, including the Serengeti, and visitors should avoid bites by tsetse flies. The AIDS epidemic is serious in the country with about 8% of the population infected with the HIV virus. Cholera outbreaks are common throughout the country and visitors are advised to avoid tap water. Food prepared by roadside vendors should be avoided.