Before coming here, visit a travel medicine doctor/clinic. Get your booster shots updated. Ask for some travelers diarrhea/Cholera prophylaxis (vaccine). Visit this doctor/clinic at least 8 weeks before arriving in the DR. You can do a lot to keep yourself from getting sick, but you cannot control what others do in restaurants, cafes, bars or even the vendor where you buy your vegetables.
If you have to peel it to eat it, you're generally ok. If not it needs to be washed well, cooked well and eaten hot, not just warm. Beware of cold salads, ice cubes and buffets where people are digging in without washing their hands. After you have been here numerous times or live here, your immune system is better able to handle some of the bugs you can easily pick up. First time/infrequent visitors or the truly naive can and do get sick, some die. The DR is not as bad as some countries, but it's not Texas either.