Cancer

They found him on the cave floor, his hands and feet melded into the living rock.

"Ah," he sneered. "The white blood cells arrive."

Rosalyn's face flickered between horror and admiration. "What have you done, Cyril?"

"An infection," he said, sinking further. "The planet can't last much longer without guidance, and since it won't take some goddamned responsibility..."

"You've failed." Grouter's voice was harsh and guttural. "We here. We can kill you."

"Yes, yes, you're the planet's immune system." Something - several somethings - hissed and clattered from the corners of the room. Shadows surrounded the invaders. "My immune system is stronger."