The nehret was a fatal transmittable disease that lay dormant for years until suddenly afflicting mature Klingons at any time. Instead of regarding this as an illness, it was referred to as "old age."
Unlike many diseases, the nehret was not filtered out by 24th century Federation biofilters. As a result, the disease was carried aboard USS Voyager 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 cure the disease when it was discovered that the unborn baby had developed an immunity to it, owing to her ¾ Human genome. (VOY: "Prophecy")