Pernicious Anemia, the inability of the body (digestive system specifically) to absorb B12, can be very difficult to detect. An ex-gf had it, and spent two years going to doctors all over the world trying to figure out was wrong with her.

At various times she was diagnosed with a hole in heart (for which invasive open-heart surgery was the cure), auto-immune disease, MS, depression, and other totally worse and completely unrelated conditions. She was suspicious of all and kept getting second opinions until she finally found a doctor who got it right. The treatment is relatively simple, weekly self-administered B12 injections.

So if you find yourself feeling worn, tired, mentally "cloudy"/"fuzzy", neuropathy (fading feeling) in your extremities, or other neuro-related issues, definitely look into this before you investigate more severe conditions.