Llhrei'sian (pronounced racy-on) was a sickness that affected Vulcans and Romulans. Humans and Byzatium could not have llhrei'sian through subspace turbulence, unlike Romulans, where a Grust 5 turbulence and higher could cause the malady. Its name, literally translated to English, meant diarrhea. In a sense, the people afflicted from llhrei'sian suffered from diarrhea, but the sickness also dulled one's senses and balance.
There were other causes to the malady but the cure for llhrei'sian was not replicable. The shortage of the remedy aboard the Bouteina almost made the crew mutiny and mandated an away mission to get some more. ("What about some D'oh!")