I posed that question on a heavily Rudi site, on a thread about Fred's lymphoma. There were lots of flames in the responses. Good thing I had on my asbestos suit.
You might also note that Senator McCain has been treated repeatedly for a malignant melanoma of the face.
Finally, please remember John Kerry had a total prostatectomy a short time before running for President. He, like all the Pubbie candidates thus far, refused to make all personal health records available to the press.
There is a squeamishness of the press and others about asking tough questions and demanding full answers—by data not by speeches—as to personal health.
Being able to physically able to do the job is further compounded by questions of psychological consequences of an ongoing, fatal illness. Wilson, FDR, JFK and, yes Reagan in his last term, all had severe disabilities which supposedly have now been dealt with by legislation. Better to assess the health care of candidates before they are elected than after! We all do get sick and die, this should not be the problem it is. There is a general reluctance to talk about one's own diseases and death--especially by those who hold high public office.