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")
In 2381, Ensign Beckett Mariner indicated that the participants of the Annual Intra Division Pie Eating Contest held aboard the USS Cerritos usually ended up with "sugary diarrhea" from consuming too much pie. (LD: "Trusted Sources")