Malaria presence on the island.
In the DR everything in dark blue away from the border provinces was recontaminated within the last decade or so. In the Punta Cana region the first malaria cases ever were reported within this decade, if I'm not mistaken. Even some of the tourists got infected. Everyone knows what demographic introduced that disease there, but most people are afraid to say anything out of fear of being called a racist.
In Haiti malaria coverage has remained pretty much unchanged since colonial times. Shows how much the Haitian political class cares about their people.
Most diseases that have been erradicated in the DR was under the Trujillo and Balaguer governments. In the governments after Balaguer the amount if diseases that were erradicated decreased. From about the mid-2000 decade, which is when the massive illegal immigration from Haiti began to become massive, diseases long erradicated started to reappear.
Haiti's government has never taken measures to rid their populations of diseases that can be erradicated. Now its impossible due to its weakness, but during Duvalier's dictatorship they could had cleaned their side of the island of many diseases but decided not to do that. Their efforts were on dismanteling Haiti's production capacity leading to a collapse in the 1980's.