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?