This quote is on pg. 254 of my edition of What To Expect When You Are Expecting.  It is a footnote and it says:  "There have been some suggestions that, in families with a history of premenopausal breast cancer, the condition might be passed on from mother to daughter through a virus in the breast milk.  This theory is unsubstantiated, and it appears that women with such family histories, and even those who have had cancer in one breast, can safely and successfully breastfeed their babies."

Since there are no references in the book, I have no idea where this information came from.  (Oh, BTW my edition of the book is the 2nd edition, published in 1991).

Heather Davis