Zika has now been declared endemic in parts of Belize, and it has been a much-discussed topic among our recent guests. We take the threat of Zika very seriously, but don't necessarily think it means you need to cancel your Belize vacation, or a vacation to any other area where Zika is endemic. You should, however, be prepared to take precautions. Use a lot of insect repellent. Plan not to get pregnant until you and your partner have both been tested for Zika after you return from your trip. And if you are pregnant, you probably shouldn't visit Belize.
The photo below shows the types of mosquitos that carry diseases. I'm not sure how helpful it is since you usually can't identify a mosquito until it stops flying to land on you and bite you, but it is interesting. I also copied the CDC's advice on how to deal with Zika in Belize, which is more helpful.