"I believe the original source is the 1988 PBS documentary ""Plagues"", which was hosted by Blumberg according to http://www.the-scientist.com/?articles.view/articleNo/9433/title/Baruch-BlumbergScience-on-TV/  You'll notice this source says, ""may have contributed to as many as half"", which is weaker than the form usually stated as fact these days.  Is this claim really plausible?   

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