Who is at risk of vitamin B12 deficiency?

Many. The classic B12 deficient people are severe alcoholics and vegan type vegetarians. Obviously, the reason for the deficiency is different in the two groups. Vegans simply don't eat enough B12 containing foods. Alcoholics are a bit more complicated than that.
Auto-immune. Dr. Level answered this as well. Besides dr. Levey's answers, B12 deficiency can also have auto-immune ties.