People who live/work in Tanzania for long extended periods... like 'life' rarely take malaria meds, but I can assure you most all of them have contracted Malaria at least once and probably more than that. Locals know how to determine whether they have contracted the disease and are immediately treated, but this deadly disease can live in the human body for life. Besides, while systems usually appear within 10/days... malaria has been known to live in the body up to a year or two before systems appear.
Malaria kills over 1/million worldwide annually; mossies who carry malaria are out and biting from dusk to dawn, whether on the mainland, but especially in a tropical environment - hot//humid - which Zanzibar most definitely is.
Malaria is not the souvenir you wish to bring home from holiday.
Discuss with a doctor or tropical diseases specialist.
Edited: 2:39 pm, December 03, 2012