In his second term as president, Dwight Eisenhower looked like an old man. He’d had a serious heart attack in 1955, requiring extensive hospitalization. Ike later suffered a stroke. In contrast to his seeming senescence, his successor, John F. Kennedy, seemed vibrant and flamboyant.

The reality was that Eisenhower was not really that old — he was just 62 when first elected. And Kennedy wasn't actually that  vigorous, and indeed was secretly afflicted by serious medical problems, including Addison’s disease, that his aides concealed from the public.