You will always want to check the CDC site or have your doctor or travel clinic check to be sure there are not any unusual outbreaks in your place of travel. This applies to Haiti or anywhere else you want to go.
Generally requirements are as follows*:
Hepatatis A (a shot)
Typhoid Vaccine (typically a shot)
Anti-Malarial Medication (usually oral medications)
*Be aware that some of these medications have to be administered a certain number of days prior to travel, so do not wait for the last minute.
Measles & Standard Immunizations:
You will want to be sure you are up to date on all standard immunizations. If you are not 100% sure if you are protected against measles there is a blood test your doctor can do to check. If they do not feel you are adequately protected you can receive an MMR booster. Try to check this well before getting your Typhoid shot so that one does not diminish the effectiveness of the other.