Depending on the severity - before launch probably, after the launch it's a risk call and they would probably adopt wait and see if it gets better approach. The

Depending on the severity - before launch probably, after the launch it's a risk call and they would probably adopt wait and see if it gets better approach. The reason for scrub would not be the diarrhea, but rather not knowing the reason why. Astronauts are placed in isolation / fed high quality and low diarrhea risk food. This mitigates most of normal reasons of why it could happen, so getting proceeding with the mission without understanding what is going on is risky.