In the dark I thought I heard somebody call

And now, b12 stuff:

It is also the only vitamin needed by humans that, to the best of our knowledge, can be gotten only from animals (or pills). It is present in milk and eggs, and meat. (However, It may also be found in nutritional yeast, a Fungi.) It is needed in only very small amounts -- RDA is 2.4 micrograms.

Pernicious anemia is caused by a persons' body being unable to absorb enough B12 to form healthy red blood cells. B12 is also necessary to maintain the sheath surrounding and protecting nerve fibers.

B12 is a co-enzyme based on cobalamin. There are two forms of B12 that work as co-enzymes (adenosylcobalamin {also called 5'-deoxyadenosylcobalamin} and methylcobalamin), but our bodies can convert most cobalamins into one of these.

While what Tem42 said is true, that vitamin B12 is only found in animal products (and is therefore a stumbling block in pure vegan diets), the human body is extremely good as sequestering this stuff.

The average adult Westerner eating a normal diet will have several years worth of vitamin B12 stored in his/her liver and other places in their body. It will take several years for the body to deplete its stores of vitamin B12 after starting a strict vegan diet.

Deficiency of vitamin B12 is a serious condition which can result in all sorts of neurological problems including weakness and cognitive deficits. Vitamin B12 deficiency is also a differential diagnosis of macrocytic anaemia.