Vitamin B12 deficiency
Vitamin B12 deficiency or hypocobalaminemia is a low blood level of vitamin B12. It can cause permanent damage to nervous tissue if left untreated long enough. Vitamin B12 itself was discovered through investigation of pernicious anemia, which is an autoimmune disease that destroys parietal cells in the stomach that secrete intrinsic factor. Pernicious anemia, if left untreated, is usually fatal within three years.