This subject has been extensively discussed. Do a search of the archives. The general consensus is that chloroquine prophylaxis is usually viewed as overkill by people going ONLY to Ambergris Caye (or other resort areas) because the risk of malaria is very very low, and particularly on AC.
If you are going to be camping in the bush, or spending considerable time outside of resort areas in Toledo, Cayo or Orange Walk districts, you should seriously consider taking the chloroquine prophylaxis treatment. There are no choloroquine resistant malaria vectors known to be in Belize. Choloroquine does not usually cause substantial side effects in most people who take it (unlike the prophylactic for choloroquine resistant malaria, which is pretty nasty stuff).
You should be prepared with a bottle of some kind of repellant containing DEET, as well as the so called "natural" stuff, though you may not need it on AC, but you won't know that til you get there. Not many mosquitos USUALLY when its not rainy, but sometimes the no see ums come out, and if you are coming in the rainy season, you will want your "Off" around twilight at least, or if going biking on a jungly path. If you want protection against no see ums, you need the "Deep Woods" version of OFF (the green label...the orange label stuff won't protect you against no see ums...)
No I am not a doctor..nor do I play one on this board (unlike certain other people who will go unmentioned)..just an experienced traveler who has done the research.
AmbergrisCaye.com 
