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It was explained in an episode of Enterprise. About that time transgenic humans made a reappearance. One of the things they did was capture a Klingon warship (the humans were stronger faster and tougher).
Klingon scientists somehow got a sample and started experimenting (because they can't stand being second best). One of the early test gene sequences has an odd interaction with a virus that the test subject happened to have. The virus mutated and went from being a cold to a plague. It could have wiped out the Klingons species.
The cure involved splicing more human DNA into Klingons. An unfortunate side effect was smooth foreheads.
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