Why do Klingons look different during Captain Kirk’s day than all the times after and before? What happened to their forehead ridges during that period? (We all know the real reason – the original show didn’t have the budget for prosthetic makeup … and starting with the movies they did. But that explanation is no fun!) … The 15th episode of Season 4 of Star Trek: Enterprise will begin to address that issue, and it’s called “Affliction.”
To be aired around Feb. 18, 2005.
Patience people, patience.
Update: The answer as explained by Wiki:
One of the test [klingon] subjects had a virulent flu, which — combined with the genetic changes wrought by the experiment — became a deadly, airborne plague that spread rampantly across the Empire, from world to world, causing the physical changes to change them into the human-looking Klingons of Kirk’s day.