Vitamin B12 was discovered from its relationship to the disease pernicious anemia, which is an autoimmune disease that destroys parietal cells in the stomach that secrete intrinsic factor. Pernicious anemia, untreated, is usually fatal within three years. Once identified, however, it can be treated successfully and with relative ease (although it cannot be cured and continued treatment is required). Intrinsic factor is crucial for the normal absorption of B12 in amounts that occur in foods, and thus a lack of intrinsic factor, as seen in pernicious anemia, causes a vitamin B12 deficiency...