Recommended Vaccines: Get your routine immunizations up to date, and do not overlook the poliovirus vaccine. Hepatitis A and typhoid vaccines should be included. The prevalence of hepatitis B is listed as "intermediate" (as opposed to moderate or high) in Haiti, so you might want to consider vaccination against it.
Malaria: All areas are at risk, and chloroquine is the recommended antimalarial drug.
Other Issues: Haiti is in the Caribbean's hurricane alley, so plan carefully if you travel between July and September. The US Department of State advises against nonessential travel to the island nation because of political instability and violence.