There are no compulsory vaccinations for Zanzibar for those coming from Europe. The Yellow Fever vaccine is compulsory only for those passing from zones notoriously infected. Malaria, tetanus, typhoid, hepatitus A, B and C are common illnesses and at times there may be cholera epidemics. More common complaints are intestinal, urinal or eye infections.
The risk of malaria exists all year round in Tanzania, Zanzibar included with higher risk during the rain season (spring and autumn). We therefore advise travellers to consult their doctor before their trip to get the best advice on vaccines or anti malarial pills.
This said, common sense should always prevail-cover up in light clothing especially in the evenings and when visiting forests or marshy areas(long sleeves and trousers). Use insect repellent and mosquitoes nets (provided on windows and beds in the guesthouse).
Pay particular attention to what you eat and drink.