Staying Healthy in Asia, Africa, and Latin America, by Dirk G. Schroeder (Avalon Travel Publishing, 2000), is an excellent and concise guide to preventative medicine in the developing world, and small enough to fit in your pack. Same goes for The Pocket Doctor: A Passport to Healthy Travel, by Stephen Bezruchka (Mountaineers Books, 1999). A bit more unwieldy but a standard in the field is David Werner’s Where There Is No Doctor (Hesperian Foundation, 1992).
For up-to-date health recommendations and advice, consult the “Mexico and Central America” page of the U.S. Center for Disease Control and Prevention (CDC) at www.cdc.gov/travel/camerica.htm [1], or call their International Travelers Hotline at 404/332-4559 or 877/394-8747. Another excellent resource is the Belize page of www.mdtravelhealth.com [2]. You can also call the Belizean embassy in your country for up-to-date information about outbreaks or other health problems.
Links:
[1] http://www.cdc.gov/travel/camerica.htm
[2] http://www.mdtravelhealth.com