Ametan rubeola was a viral disease discovered sometime prior to the mid 23rd century. Its symptoms included dehydration, and often led to pnuemonia and encephalitis. It spread quickly, and if left unchecked, could decimate entire populations. Federation medicine developed a vaccine which kept the virus dormant in humans.
Circa 2265, a dormant virus in the system of by Dr. Quentin Melander mutated when exposed to the atmospheric radiation of planet NGC 667, and spread to the native population of Grennai. Melander died of the mutated virus, as did the entire population of the Grennai village under Federation cultural observation, a total of 238 beings. Melander's colleague, Dr. Revati Jendra, returned to NGC 667 circa 2268 to help prevent the virus from spreading to the wider Grennai population. (TOS short story: "First, Do No Harm")