Nehret
The nehret was a fatal transmittable virus that lay dormant for years until suddenly afflicting mature Klingons at any time. The Klingons themselves never even considered it might be an actual illness, instead believing it was simply "old age" that kills those unlucky enough to not die in combat.

Unlike many other diseases, the nehret was not filtered out by 24th century Federation biofilters. As a result, the disease was carried aboard  by a ship of Klingons who had traveled for over a century to the Delta Quadrant on a quest for the fulfillment of the kuvah'magh prophecy. At that time, B'Elanna Torres and her unborn child were infected. The Doctor was able to synthesize an antivirus to cure the disease when it was discovered that the unborn baby had developed an immunity to it, owing to her ¾ Human genome. ()

de:Nehret
Medical conditions
Qo'noS