Though never explained in the film, the fact she was found in an abandoned mental asylum suggests she may have been a patient that had a number of experiments performed on her. However it can be taken from a number of clues that she may in fact be a zombie. The strongest evidence for her being a zombie is the complete lack of sentient awareness and knowledge that a human possesses and that her bite reproduces her condition in the victim, both of which are considered classic traits of a zombie. Other classic zombie behaviour such as homicidal violence is also observed.
One aspect of the film that casts the zombie theory into doubt is the disparity between Joann and Johnny being bitten. Joann's transformation is almost immediate whereas Johnny takes a number of days to change and even seems to retain full awareness. This however may simply be a plot convenience, similar conveniences have been used in other more traditional zombie films.