Travellers are advised to take medical advice at least three weeks before leaving for Tanzania. Most visitors will need vaccinations for hepatitis A, typhoid, yellow fever and polio. Those arriving from an infected country must hold a yellow fever vaccination certificate. There is a risk of malaria all year and most doctors will recommend medication. Sleeping sickness is a risk in the game parks and visitors should avoid bites by tsetse flies. HIV/Aids is a serious risk. Cholera outbreaks are common throughout the country and visitors are advised to drink bottled or steralised water only. Medical services are available in Dar-es-Salaam and other main towns, but facilities and supplies are limited; visitors with particular requirements should take their own medicines. Comprehensive medical insurance is advised.