Doppelganger

After coming across a crystalline species on M3X-387, similar to one which Jack O'Neill encountered in the SG-1 season 1 episode "Cold Lazarus",[4] Sheppard inadvertently carries it back to Atlantis after contact with it. The being can travel through electrical conductors, and can enter the minds of humans through touch, where it causes nightmares - which eventually kill Dr Kate Heightmeyer.