If the Democratic and Republican presidential nominees stick to their latest promises, we may learn more about their health histories in the next few days.

The pressure had been building for more information and only intensified when it was revealed Sunday that Hillary Clinton had been diagnosed with pneumonia. But the disclosure of personal health information is a relatively recent phenomenon. Withholding the details of personal medical histories is a political tradition that dates back to the founding of the republic.

"When you look back at the presidents and how they have managed information about their health, I don't trust any of them, to be honest with you – beginning with George Washington," says historian and journalist Matthew Algeo, author of "The President is a Sick Man," an account of how President Grover Cleveland's inner circle kept his cancer surgery secret from the public for years.

President Washington's legendary candor didn't extend to a large and painful tumor that was surgically removed from his left thigh in 1789 without anesthetics or antibiotics, Algeo says. The president was critically ill for days after the surgery, and his work had to be done by secretaries. It took him two months to fully recover. Yet, the state of his health was closely guarded within his inner circle, Algeo says. "He never told anybody about it."

By then, Washington was already in office, busy with Congress, and at arm's length from the public. But his health history was equally inaccessible to voters before his election. That's because voters rarely got to see candidates campaign in person throughout most of the 18th and 19th centuries.

"It was considered unseemly for them to go out and make speeches for themselves, where their health could be observed," says University of Pennsylvania political scientist Rogers Smith. "It was largely a matter of rumor."

After presidential candidates hit the campaign trail in the late 19th and early 20th centuries, their infirmities were often shielded from the public. Franklin Delano Roosevelt, hobbled by polio, hid the extent of his disability. "While campaigning there was concern that presidential candidates appear robust," Smith says. "FDR used leg braces so that he could stand on his own, and he avoided being photographed in a wheelchair when he was campaigning. Most Americans did not understand the degree of his disability."

Even as late as the 1960s, a "gentleman's agreement" with the media allowed presidential candidate John F. Kennedy, whose appeal at age 46 partly stemmed from his "youthful vigor," to conceal the severity of a World War II back injury and his heavy dependence on medication, Smith says.

Once in the White House, presidents' health problems are even more closely guarded, President Abraham Lincoln got smallpox, a fact never shared with the public. President Woodrow Wilson suffered a stroke that left him essentially unable to govern for the last 18 months of his second term. President Warren Harding hid heart disease that ultimately took his life while in office.

Perhaps the most colorful incident was Cleveland's cancer surgery. In the summer of 1893, Cleveland disappeared for four days during his second term as president to undergo secret surgery on a friend's yacht cruising up the Hudson River from New York City to his summer home in Cape Cod, Massachuestts. The surgery, to remove a large tumor from Cleveland's palate, was done in a special chair, so that the team of six surgeons could gain access to the cancer through the president's mouth and spare his elegant moustache.

The surgeons removed a large section of the president's palate, jawbone and five teeth. They created a prosthesis that would allow Cleveland, once his wounds healed, to eat and speak normally. When the yacht, the Oneida, arrived in Cape Cod, Cleveland disembarked in secrecy.

"Cleveland's people kept all the reporters off the property, so they wouldn't see him get off the boat and walk with assistance into the house," Algeo says. "If they had had telephoto lenses back then, Grover Cleveland never would have gotten away with it."

He had another advantage that doesn't exist today: In the 19th century, Washington, D.C., emptied for the summer. It wasn't unusual for a president to go to a summer home for two or even three months. "He had two months to recover," Algeo says. "By then his wound had healed enough so that he could speak at official functions. It would be impossible for a president today not to utter a word for two months, but that's what Grover Cleveland did in 1893."

With non-stop media coverage, secrets are all but impossible to keep these days. Modern campaigns -- and presidential administrations -- are gruelling, 24/7 marathons with little time to rest and recharge. The ambition, drive and exertion required of politicians and other high-level public figures takes a toll.

In the mid-1950s President Dwight Eisenhower had a serious heart attack, and while his aides acknowledged what had happened, they downplayed its severity. Astonished viewers in 1992 saw President George Bush collapse at a state dinner in Tokyo, an incident attributed to the flu -- and Bush's failure to follow doctors' orders. In 2010, U.S. Army General David Petraeus momentarily fainted on live television while giving congressional testimony.

Still, cameras can't reveal everything. Historians still debate whether president Ronald Reagan began showing signs of Alzheimer's during his second term, a prospect kept from the public.

There are, however, exceptions -- politicians who, in response to public pressure, do share their personal medical histories with the public. In 2000 and 2008, Sen. John McCain, seeking to become the nation's oldest president (he was 71 during his second campaign), released hundreds of pages of medical records to counter a "whisper campaign" about his fitness to serve. The records disclosed arthritis and melanoma, both stemming from his service in Vietnam.

The current candidates' age -- Clinton is 68 and Donald Trump is 70 -- also makes questions about their health particularly relevant, says the University of Pennsylvania's Smith. "The fact that we have the two oldest candidates ever running for president does raise questions about the conspiracy of silence about the health of presidents."

Sen. Bob Graham, D- Fla., believes politicians must be open with their constituents. Graham made public medical records spanning 23 years during his 1986 Senate campaign. The records disclosed that he had a simple phobia about blood that ratcheted up his anxiety during medical exams.

He says the phobia vanished in the early '70s, after he spent a day riding around with police to see what the job was like. They were called to the scene of a bloody shooting, where they had to take care of the victim and secure the crime scene.

"A visit to the doctor is a lot less traumatic than that," he said at the time.

Today, Graham tells U.S. News that the release of his medical records was a "non-event" that did not affect his campaign at all.