GENERAL RECOMMENDATIONS ON ENTERING THE COUNTRY.
Visas obtained from Tanzania are required for all visitors except members of the Commonwealth or Scandinavians. The government will very soon implement a flexible system of granting visas at points of entry. When entering Zanzibar even from the mainland you will be required to pass brief customs and immigration formalities.
Check with your local consular authorities as to which vaccinations are required (normally just yellow fever). Prevention against malaria is highly recommended and should not be taken lightly.
Malaria can be dangerous under certain forms and when back in your country, if you feel ill at any later time and the diagnosis is uncertain, remember to inform your doctor that you spent sometime in a malaria-endemic region; it may help ti identify the cause of your condition. Before departure ask which medication is best suited for you, it is always better to go to a hospital familiar with tropical conditions.
Medicine are available but may not be of the same kind you are used to. Drinking non-bottled water invite possibly unpleasant experiences. Hospitals do not have proper equipment to treat major case.
There is no reason to be overly anxious about security. Disregard for elementary common-sense precautions may cause merely the same inconveniences you might incur anywhere else. Police stations are numerous and you will probably receive better attention than you would at home.