They explained the "smoth head" Klingons in the Enterprise series. It was a result of human augmented DNA(remember Khan) combined with Klingon DNA and mutated because of Tykellian Flu(I think Tykellian, but it was a flu strain). The cure caused those Klingons infected to lose their cranial ridges. It was a widespread virus, so that explains how we saw "smoothies" in TOS.

They obviously "got better" because Koloth, Kor, and Kang had ridges on DS9 when they appeared.