Scans_Daily

IIRC it was implied to have been a left over side effect from what Fabian Cortez did to the team in the first few issues. His screwing with their powers caused a lot of weird shit like Kitty gaining one of Wolverine's claws, his healing factor and the ability to grow razor sharp finger nails out of her other hand.

Add to that the fact that in forever Claremont was recycling one of his favorite ideas the implication that mutants powers are doomed to kill them some day with only a few mutants like mystique and Mr. Sinister in his real idenity as the immortal twelve year old stalker of Scott Summers possibly being naturally immune.

In fact IIRC this whole situation was originally set up by mystique to try to share Kurt's potential inherited immunity with Rogue and it kind of back fired.