On the one hand, we have the case of FDR. Roosevelt was mostly confined to a wheelchair because of polio. It was for vanity reasons that the press covered up photographing him in the chair. As we know from his four terms, it wasn't a health issue.
On the other hand is the 1992 case of Paul Tsongas. In the 1992 Democrat primary, former Massachusetts Senator Paul Tsongas was the early front-runner; Bill Clinton was struggling with infidelity questions. Tsongas had retired from the Senate in 1984 because he was diagnosed with non-Hodgkins lymphoma (John Kerry took his seat). Tsongas underwent a bone marrow transplant to treat the disease in 1986 and received a clean bill of health from doctors in 1991.
The whisper campaign began against Tsongas that his health was still in doubt, and Tsongas' popularity began to wane as Clinton's begun to surge. Tsongas eventually dropped out of the race.
As it turned out, Tsongas died 1997 from a return of the cancer, which would have been in the seventh year of his Presidency. People were right to question his health.
Stories like this one are of the whisper campaign kind. They point to real health questions that the candidate wants to hide, but are real concerns.
-PJ
Obviously, keeping from the public Roosevelt's paralysis from polio and reliance on a wheelchair saved his political career and changed the course of American history.
BTW, by Roosevelt's fourth and final presidential campaign in 1944, the press was covering up even more serious medical issues: Roosevelt was known (in inner circles) to be near death because of congestive heart failure. The wiser decision at that point might have been to step down after three terms, and let a more physically capable successor replace him. But Roosevelt ran and won, then died less than three months into his fourth term. The public, by and large, had no idea of his condition because the media never mentioned it.