The fact remains that a deficiency is statistically far more
common with most nutrients. Drugs and disease states
deplete nutrients on a daily basis. Also people may have
a genetic error in the metabolism of some vitamins.
B-complex is often this target. Some people are even born with a B6 error, which causes seizures. It is called infantile spasms, and these patients require far more B6 to live normally than others.

Selenium poisoning occurs with livestock from eating plants that concentrate selenium from water runoff. Also feed can be accidentally poisoned by adding the wrong amount when it is compiled.

This article is pretty good. It describes how some areas of the world actually have selenium depleted soils:
http://healthlink.mcw.edu/article/964647329.html
(this article is a copy from NIH)

If you are not taking any vitamins, it is important to find the source of your selenium. I put up an article on another thread here about various hidden sources, from dandruff shampoos, to beer.