He confided in his close friend, the White House physician, Cary T. Grayson: "My personal pride must not be allowed to stand in the way of my duty to the country. … If I am only half-efficient I should turn the office over to the vice president. If it is going to take much time for me to recover my health and strength, the country cannot afford to wait for me."

The doctor talked Wilson out of it.

"Grayson is really taking the lead in the coverup here" while acting in accordance with First Lady Edith Wilson's wishes, said Patricia O'Toole, a biographer who is working on a book about Wilson.

Wilson's illness is the most remarkable example in U.S. history of the public being kept in the dark about a president's health troubles. As letters, oral histories and Grayson's own diary showed, there was a concerted effort among the doctor, Edith Wilson and members of the president's inner circle to minimize the severity of his condition.

The nation's 28th president was having trouble persuading his political enemies in the Senate to ratify the Treaty of Versailles, the document that ended World War I. A two-thirds majority vote was needed to approve the treaty, which also provided for the creation of the League of Nations. To drum up public support, Wilson embarked on a whistle-stop tour of the country. Three weeks into the tour, on Sept. 25, 1919, in Pueblo, Colo., Wilson collapsed from exhaustion.

The train raced back to Washington, but on Oct. 2, Wilson was stricken by a massive stroke that paralyzed his left side. For days he could not move. Eventually, he recovered some ability to walk, but he was debilitated for the final 17 months of his presidency. Wilson's secretary (what we would today call the chief of staff), Joseph P. Tumulty, and Wilson's Cabinet, ran a sort of "holding pattern" presidency, O'Toole said.

'There was a question of what's in good taste'

In those days it was possible for a public figure to stay on the sidelines for months on end without it causing much concern. "There was a question of what's in good taste and what's not in good taste, what would be reckless speculation," O'Toole said. Edith Wilson is sometimes called the first female president, because she shielded her husband from most visitors. Any request had to go through her.

"The word went down that if you had a question for the president, you had to write it up briefly, and it had to be … answered with a yes or a no," O'Toole said.

Wilson's inability to passionately argue on behalf of the Treaty of Versailles doomed it.

"There was important stuff, quite in addition to the fight over the Treaty of Versailles, that needed to be taken care of that just wasn't taken care of" between Wilson's stroke and the end of his second term, in March 1921, O'Toole said. He died Feb. 3, 1924, at 67, from complications related to his stroke and heart problems.

'Clandestinely removed while sailing'

Here are other examples of undisclosed, or undiagnosed, illnesses of presidents:

Abraham Lincoln: What friends simply described as "melancholy," we know now are the classic symptoms of depression, from which Lincoln suffered most of his life, even before the Civil War and the deaths of two of his sons. Lincoln "often wept in public and recited maudlin poetry," author Joshua Wolf Shenk wrote in the Atlantic in 2005. "He told jokes and stories at odd times — he needed the laughs, he said, for his survival."

Grover Cleveland: According to a 2011 post on LiveScience.com by Maureen Salamon, Cleveland discovered a lesion on the left side of his palate "that was said to be cancerous." This was in the middle of the 1893 financial panic. "The president had the tumor clandestinely removed while sailing on the yacht Oneida to his summer home. The crew was sworn to secrecy."

Franklin Roosevelt: During FDR's 12-year presidency, the media helped him and White House officials hide his polio-induced paralysis by photographing him only from the waist up.

John F. Kennedy: Kennedy had a variety of health problems, including debilitating back pain and Addison's disease, which caused physical weakness and jaundiced skin. JFK took as many as 12 pills at a time.

Ronald Reagan: After John Hinckley's failed assassination attempt on March 1981, the White House, and the media, didn't want to project a sense of confusion or weakness. So the president was portrayed as bouncing back right away. He went home from the hospital after 13 days, but it took him about seven months to fully recover. Reagan, who was 17 days shy of his 78th birthday when he left office in January 1989, sometimes had trouble remembering details while in office. Reagan announced in 1994 he was suffering from Alzheimer's disease. He died 10 years ago, on June 5, 2004.