While the Cayo normally has relatively few mosquitoes, thanks to the porous limestone terrain that doesn't allow water to stand in puddles, there are occasional outbreaks of dengue fever. Dengue, which causes flu-like symptoms, and, in more serious cases, death from internal bleeding, is transmitted by the bite of Aedes aegypti and Aedes albopictus mosquitoes. These species are most active in the early morning and late afternoon. Travelers, especially during the rainy season, should consider using insect repellent with DEET. There is no preventative medicine for dengue.
Health standards in the Cayo are high. The water in San Ignacio and Santa Elena comes from a treated municipal system, so it's safe to drink, though many people prefer the taste of bottled water. Resorts in the region have their own safe water systems.