Vegan choriomeningitis was a disease.
In 2268, the leaders of Gideon attempted to infect their people with the disease to reduce their population. (TOS episode: "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 novel: Time's Enemy)
In 2375, there was an outbreak of Vegan choriomeningitis on Lysia. (TNG novel: Doors Into Chaos)
Information about the disease could be found in the USS Voyager's computers by 2376. (VOY video game: Elite Force)