From the U. S. State Department:

The Department of State has issued this Travel Warning to inform U.S. citizens traveling to or living in Haiti about the security situation in Haiti. This replaces the Travel Warning dated January 20, 2011 to consolidate and update information regarding the critical crime level, renewed cholera outbreak, lack of adequate infrastructure - particularly in medical facilities, seasonal severe inclement weather, and limited police protection.

The Department of State strongly urges U.S. citizens to consider carefully all travel to Haiti. Travel fully supported by organizations with solid infrastructure, evacuation options, and medical support systems in place is recommended and preferable to travel in country without such support structures in place. U.S. citizens traveling to Haiti without such support have found themselves in danger in the past.

U.S. State Department Warning
American Red Cross Haiti Relief and Development