Been to DR 8x. Had ear infection first visit, but other than that, no issues.
Keep in mind DR is one of the poorest countries on the planet. They can't afford the conditions we take for granted.
A few common sense precautions is all it takes - eat cooked food only, bite into lime (kills certain bugs), brush teeth with purified water, keep mouth closed in shower, and if you really want to be cautious, drink a bit of pepto bismol each morning to coat your stomach lining.