I always just figured hat the Klingon Empire was, ya know, a vast interplanetary Empire, made up of many planets inhabited by many races. It just so happened that our dealings with the Klingon Empire during TOS times only involved the smooth-headed peoples from one area of the Empire. Later, during TNG times, those people were in disfavor with the military of this vast totalitarian Empire, and we only saw the ridge-headed peoples. I didn't need any stupid virus to explain it in a convoluted story that awkwardly forced elements together from various other productions. The US Military is made up of people of every race on Earth. Starfleet crews are made up of every race and species from hundreds of planets. Why would the Klingon Empire's military be only one species and race?