I've read that undercooked meat can lead to intestinal parasite but not sure what they class as undercooked meat because a fair majority of steak eaters eat it medium towards medium rare and I'm sure not everyone in that category has worms.
Medium is best. If it can't be done exact tend to go for slightly well done over rare. You want to hold some of the moisture and juices but a good cut will generally hold its flavour. Can't eat a steak too rare.