Slate has a fascinating article on FDR's health leading up to his death in April 1945. He is commonly known to have been ill in 1944 during his last campaign but his problems were hidden from the public just like his polio was covered up. The public knew he had polio but they never saw how bad it was.

The author Barron H. Lerner claims that Roosevelt died of cancer. Roosevelt had a melanoma, a deadly form of skin cancer, which spread to his brain. Earlier photos show a skin lesion above his left eye, which later seems to disappear. See below. He also had congestive heart failure and hypertension. In other books I've read, he was in denial about his health. Howard G. Bruenn, his cardiologist, may have focused on his heart and missed other problems. Photographs of Roosevelt in this period show a dramatic weight loss. At Yalta in February 1945 he looks like death warmed over.
 

The new book on this subject is Eric Fettmann and Steven Lomazow's FDR's Deadly Secret. It will be released in January. Got to be a must read.

The actual state of Roosevelt's health was covered up by his doctors, and concealed by historians. If he had stepped down in 1944, Tom Dewey would have presumably won, Truman would never have been president, and the Cold War would have unfolded differently.