In 1992, then front-runner Paul Tsongas said the same thing, that he was cancer-free. The Clintons began a whisper campaign over Tsongas' health and he ended up dropping out, with Clinton ultimately winning.
Tsongas died 7 years later from a recurrence of his cancer, in what would have been his second term.
-PJ
All I remember about Tsongas, was when Al Franken portrayed him in the classic SNL skit where all the Democratic candidates were speaking at a Star Trek Convention.
Yep, died at age 55, Sanders health is going to be a major issue if he beats out mini mike.