Yellow fever
A yellow fever vaccination is only mandatory if you are arriving from a yellow fever endemic area. This includes mainland Africa if you had a layover there of more than 12 hours (so be careful if you are in Ethiopia or Kenya, as checks may be stricter). It is not officially required for direct flights from Europe, but it never hurts to have your vaccination certificate handy.
Your tetanus, diphtheria, and polio vaccinations should be up to date.
Some travelers get vaccinated against hepatitis A (and possibly B) as well as typhoid, since these can be transmitted through food or water.
Malaria
Zanzibar is considered a malaria area, although the numbers have decreased significantly due to intensive control measures.
Many Europeans take Malarone (tablets) or similar medications as a preventative measure, or at least carry them as an emergency medication (standby) in case they develop a fever. You should discuss this with your doctor individually (depending on the length of your trip and your fitness level).
Mosquito repellent (prophylaxis) is your most important protection, because mosquitoes transmit not only malaria but also the (less common) dengue fever.
DEET is your friend: Forget gentle, organic sprays. You need insect repellent with at least 30% to 50% DEET (e.g., Nobite or Anti-Brumm Forte).
Pay attention to your clothing, especially at dusk and dawn (morning and evening). You should wear long, light-colored linen trousers and socks. Mosquitoes love dark ankles!
Sleeping: Make sure your bed has an intact mosquito net. A small travel-sized vaporizer plugged into the wall socket can also work wonders in the room.
Stomach and Sun
Most health problems on the island are not tropical diseases, but rather classic travel-related ailments:
Sun: The sun at the equator is often drastically underestimated. You’ll get sunburned even in the shade or on cloudy days. Pack SPF 50+.
Gastrointestinal: “Cook it, peel it, or forget it.” Never drink tap water (not even for brushing your teeth if you have a sensitive stomach). Ice cubes in local bars are often made from tap water—when in doubt, opt for “no ice.”
Electrolytes: Pack a few electrolyte packets (Elotrans or similar). Heat and unfamiliar food can quickly deplete your body’s resources.
First Aid
In case of injuries caused by sea urchins, in addition to consulting a doctor, the tips on the information page about sea urchins can help.