Their "perfect" human bodies had no immunity whereupon they picked up a local bug on the planet and they all died.What do you suppose happened to Rojan, Kelinda, Tomar, Hanar, and Drea after Kirk and crew returned them to that planet?
The way the episode ended, it left itself open to the possibility that we would see the Kelvans again, after they colonized the planet and started a new life.

Then the Kelvan Empire never hears back from their scouts dispatched to that little neighbouring galaxy and figure it must be some really badass place to kill off Kelvans and so they decide they ain't never goin' there.
