Health: Yellow fever vaccination certificates are required by anyone arriving from infected areas, and immunisation is recommended for all visitors travelling outside the main cities. No other vaccinations are required for entry to Nigeria, but there is a high risk of malaria, typhoid and hepatitis (A & B) and visitors should seek the latest medical advice on inoculations at least three weeks prior to departure. At present there is a high risk of polio infection in northern Nigeria, as well as regular outbreaks of cholera and meningitis. HIV/AIDS is prevalent. Nigeria has the world's highest rate of Onchocerciasis, or river blindness, which is caused by black flies that breed in fast-flowing rivers. Bilharzia, dengue fever and sleeping sickness also occur.