The Klingon augment virus was an accidental plague created when Klingon scientists tried to copy human Augment enhancements in 2154, and it explains the smooth foreheads of many 23rd-century Klingons.
After learning of the Augments, the Klingon Empire sought its own enhanced warriors using genetic material from Augment embryos. The early test subjects did gain strength and intelligence, but they then died, and when one became infected with a virulent flu the altered genetics combined with it into a contagious virus.
The virus spread rapidly through Klingon colonies and threatened millions. Dr. Phlox, forced to help the Klingon doctor Antaak, developed a cure that halted the disease, but survivors and their descendants lost their cranial ridges and some of their physical traits.