We used to live in the DR and we go there frequently to help with medical teams. From the public health viewpoint, even the Dominicans suffer from gastro-intestinal infection and infestation. A big problem is the way that water is stored and delivered, in poor aqueducts and then in underground cisterns with no means of protection from contamination. Contaminated water will contain bacteria, parasites like amoeba and girardia and worms.
Precautions are found in the websites for travellers. Buy bottled water, stick to good eating establishments to be sure dishes are washed and dried well, etc., and be sure any ice is from commercial bags. Don't open your mouth in the shower. Our last trip: no one on the team became ill.
Breakfast Buffets -- get there early in the morning to avoid fly-contamination over time. If you see flies on your food, don't eat it. People are afraid of cockroaches and not flies. Cockroaches just hang around your nice, clean kitchen. Flies go from the pit toilet to your fresh mango. Flies are nasty.
Try to get a RX for Cipro before leaving the states. If you get cramps, diarrhea etc. take one, twice a day, five days. In the DR, find a pill called albendazole, for worms, take one. We have never taken a worm pill and have not had the need.
At the beach, don't swim near any river outlet. These carry polluted water.