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.
-PJ