Yellow Fever is not endemic to Belize. If you are coming to Belize from an area where Yellow Fever is endemic, you need a Yellow Fever vaccination card.
I spent some time in Africa and saw quite a few people get malaria even though they were on Larium or whatever. Malaria is really easy to treat, and the prophylaxis has some negative side effects (nausea, scary dreams, or heightened sensativity to the sun). Just remember to get a malaria test at the first sign of any symptoms, and you'll be fine.
Of course, I'm not a doctor, so don't pay a word of attention to what I have to say.