Has anyone considered that Carla might have manifested the attacks herself out of her own mind?
I mean the mind itself can make all sorts of things happen to our bodies like disease symptoms and stuff. I'm thinking she created this herself much like the ID monster in Forbidden Planet. Only it was real to her.
This is the first thing anyone thinks, including in the film itself. At least one scene consists of people discussing this possibility, and Carla points out that the bite marks are in places she could not reach precisely because she knows people are going to think this.
The possibility is ruled out in the first half of the film.