Not saying he is guilty, but if we are talking about straight up doping (bumping red blood cell counts) then it is entirely plausible that he could have doped every race and still never failed a test.
Wiki has this on doping:
At present there is no accepted method for detecting autologous transfusions (that is, using the athlete’s own red blood cells)
That said, I read one article that said people are suspicious simply because he was able to win in a sport rife with doping... ergo if *everyone* cheats to win then anyone who wins must cheat.
Doesn't take away from his battle with cancer and I don't know that he is any more guilty than his rivals of anything... kinda sad really.