I remember reading about the Cholera pandemics, with those figures, in one of the "apostate" books. I think, If I'm not mistaken, that it was Sign Of The Last Days - When? by Carl O. Jonsson, If you don't have that book it's highly recommended that you get it; it's a treasure trove of information tailored to rebut the Watchtower.
The book included primary sources in the footnotes which might help you to track the original book that made the statement.