Diarrhea, also colloquially known as "the runs", was the condition of having loose or liquid bowel movements.
Diarrhea could be a side effect of some vaccines. In 2152, Phlox warned Charles Tucker III that an inoculation he was going to administer against a lymphatic virus would cause diarrhea, as well as headache and nausea, although they would only last a few days. Tucker was apprehensive, commenting, "I'd hate to be responsible for giving the crew the runs." (ENT: "The Seventh")