Answer: Because vitamin D can't be patented and sold as "medicine." You can make it yourself. If you want more vitamin D, you don't even need a doctor, and doctors tend not to recommend things that put them out of business.
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** Update 1/21/10***
Drug companies have LIED about the H1N1 vaccine.. See the following article-
WHO and Big Pharma: Guilty of Blackmail and Extortion in Swine Flu Fiasco?