Vegan choriomeningitis was a disease. In the year 2268, the leaders of Gideon attempted to infect their people with the disease to reduce their population. (TOS episode & Star Trek 6 novelization: The Mark of Gideon) Around 2369, Morn contracted Vegan choriomeningitis, although it was highly unusual that he had. Julian Bashir was able to treat Morn and rid him of the disease. (DS9 - Invasion! novel: Time's Enemy) In 2375, there was an outbreak of Vegan choriomeningitis on Lysia. (TNG - Gateways novel: Doors Into Chaos)