Nehret

The nehret is a fatal transmittable disease that lays dormant for years until suddenly afflicting mature Klingons at any time. Instead of regarding this as an illness, it is referred to like "old age." The Doctor was able to cure the disease when it was discovered that the unborn baby of B'Elanna Torres had developed an immunity to it, owing to her ¾ Human genome.

This information taken from http://memory-alpha.org/