Aka TK or Ken at NASA - but call him the Man Who Never Caught Rubella.
Mattingly was slated to fly as CMP on Apollo 13; but when Charles Duke, Captain of the backup crew, fell ill with Rubella, Mattingly was found to have no immunity. Not wanting to risk a sick CMP during a Moon flight, NASA removed Mattingly from the roster and sent Jim Swigert instead.
Mattingly, deeply depressed about his removal at first, turned out to be one of the most important assets in the saving of Apollo 13, working feverishly to work out the rescue plan.
He got his moment in the Sun as CMP of Apollo 16, the mission that Swigert would otherwise have been assigned to. He did one quite singular feat during that mission, becoming the first human to EVA in "deep space", retrieving, during the cruise back to Earth, photographic film that had been exposed in an automatic Moon monitoring experiment located on the body of the CM.
Mattingly later also flew on two Shuttle missions. And he never fell ill with Rubella.
Godspeed, TK