ROMANS-SUR-ISÈRE, France—Two weeks into the biggest race on his calendar, pro cyclist Pierre Rolland was spending evenings in what he called Tour de France quarantine. At team hotels, he bunked alone in his single room. At meal time, he ate by himself or at the end of a table. When teammates were around, he was banned from shaking hands.

No one felt good about it, but he knew that cycling’s answer to solitary confinement was the only solution at times like these. Because Mr. Rolland fell prey to one of the sport’s greatest...