Trivia: A constant question during the run of all the Trek series is why Klingons look so much different from Star Trek: The Motion Picture on than they did in the original series. The real reason is the movies and later TV series had a better makeup budget. However, the Star Trek: Enterprise episodes "Affliction" and "Divergence" provide a canon answer. Klingons acquired genetically engineered human embryos left over from Earth's Eugenic Wars and used them to augment their soldiers. It worked but created a virus that threatened to annihilate the Klingon race. Dr. Phlox and a Klingon doctor found a cure, but it resulted in all Klingons becoming far more human in appearance. Sometime between these episodes and the first Trek movie a cure was found, returning the Klingons to their present day "ridged-head" appearance.
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