A little information about B12, from "Could It Be B12? An Epidemic of Misdiagnoses" 2nd edition by Sally M. Pacholok, RN BSN and Jeffrey J. Stuart, DO. (A very good book, and not too technical, by the way. It's about B12 deficiency in humans, but I don't see why it wouldn't apply to cats, dogs and other mammals as well.)
As B12 deficiency continues, your immune system also falls prey, because it can no longer produce enough disease-fighting white blood cells. Thus, you become an easier target for viral or bacterial infections. Your gastrointestinal system suffers as well, because your body can't make enough cells to replace your intestinal lining efficiently, so you may experience diarrhea, nausea, or severe appetite loss. And eventually, as your B12 deficit grows, you're likely to feel exhausted and weak due to the anemia that occurs when your body can't make enough healthy blood cells to carry oxygen to the cells of your body.