The CDC recommends anti-malaria drugs for short-term travelers. We took them on our shorter trips during our adoption. The side effects were not so good. We were never big fans.
Our Dr. advised against anti-malaria meds for long term use. He said the damage that could be done by using the meds long-term could be worse than getting and dealing with Malaria.
So, to answer the questions, that is why Troy (and none of us) are on an preventative medicine. Malaria is common in tropical climates. Haiti has more of a struggle with it than the Dominican Republic because the deforestation here means that the water has little opportunity to be absorbed when it rains. Rather than absorbing, it sits stagnant for days or weeks and becomes the perfect breeding ground for mosquitoes.
Clear-cutting your land is a bad idea, all around.