The Fix's Aaron Blake explains the incident during which Democratic presidential nominee Hillary Clinton fell ill on Sept. 11, and why her health is likely to remain a subject of discussion. (Peter Stevenson/The Washington Post)

When it comes to personal health, Hillary Clinton has long opted for intense privacy, dating back to 1998 when she experienced what she later described as her “scariest moment” with a blood clot in her right leg.

At the time, President Bill Clinton’s White House said little about the first lady’s medical emergency, discreetly treated as an outpatient at Bethesda National Naval Medical Center. Hillary Clinton, who told the White House physician then that she was too busy campaigning for Democrats to be admitted to the hospital, later described the incident as “the most significant health scare I’ve ever had.”

Doctors found a second clot in her legs in 2009, a fact not widely known until her Mount Kisco, N.Y., internist released a letter in July 2015 attesting to her good health as she launched a campaign for the Democratic nomination for the presidency.

Clinton’s third blood clot in 2012 — this time in her skull — was too serious to keep under wraps, since she was serving at the time in a high-profile role as President Obama’s secretary of state. She spent several days in New York-Presbyterian/Columbia University hospital, and her husband later said it took her six months to recover.

Clinton’s privacy about her health has continued even as she enters the final stretch in a run for the presidency, with her Republican opponent Donald Trump suggesting, without specifics, that her health could pose a problem if she is elected.

Hillary Clinton exits her daughter Chelsea Clinton's apartment at 21 E. 26th St. after attending an anniversary event at the World Trade Center 9/11 Memorial in Manhattan, NY. (Yana Paskova/For The Washington Post)

After nearly collapsing at a 9/11 memorial service in New York on Sunday, Clinton’s doctor revealed that the candidate has pneumonia, which was diagnosed last week and is being treated with antibiotics. The combination of heat and a powerful antibiotic, the doctor said, could explain the candidate’s wobbly steps as she was whisked into a black van.

Now, the mysterious episode in New York has sharpened the focus on the health of the two major party presidential nominees and led to a growing clamor for the release of more complete medical histories. Trump has been criticized for releasing even less health information than Clinton.

“This is the most important job in the world, and we have two old people [running for it],” said David Scheiner, who was Barack Obama’s physician for more than two decades until his election in 2008. “The plane doesn’t work as well at 70 and 68 as when you’re 45 or 50. . . . Bad things happen.”

At 68, Clinton’s discretion about her medical history has made it difficult for voters to assess her complete health picture.

She has deep vein thrombosis, which requires her to take a daily dose of a blood thinner that needs frequent testing. Her personal physician, Lisa Bardack, disclosed last year Clinton’s treatment with the drug Coumadin.

Sunday’s episode illustrates how those closest to Clinton handle news about her health. At first, a Clinton campaign spokesman said Sunday that Clinton “felt overheated” at the 9/11 event. Later, Bardack painted a fuller picture, saying in a statement that Clinton had been diagnosed with pneumonia on Friday and became “overheated and dehydrated” two days later.

Doctors say “heat stress” illnesses ranging from cramping to fainting are relatively common in older adults and not a concern as long as the symptoms are fleeting. And they can certainly be exacerbated by illnesses such as pneumonia.

Video of Clinton’s departure seemed to show her buckling and stumbling as she got into her van. (Thomas Johnson/The Washington Post)

“It’s usually not indicative of broader health issues,” said James Goodwin, chairman of geriatric medicine at the University of Texas Medical Branch.

Clinton’s history with blood thinners does suggest that doctors might want to conduct further testing, doctors told The Washington Post.

“You always have to be worried about internal bleeding,” said Samuel Durso, director of geriatric medicine at the Johns Hopkins University School of Medicine.

The 1998 episode came during a fundraiser for Sen. Charles E. Schumer (D-N.Y.), Clinton recalled in her memoir “Living History.” Her right foot was swollen. She could barely put on her shoe. At the hospital, tests revealed “a big clot” behind her knee. Clinton briefly took blood thinners and the condition went away.

Clinton in her memoir attributed the clot to her “nonstop flying around the country.” Former vice president Dan Quayle also had deep vein thrombosis from flights.

Little is known about the 2009 clot episode.

A risk factor is sitting on plane flights for longer than six hours, which is one reason business travelers walk up and down the aisle during long trips. The condition, while treatable, can be fatal if the clot moves to the lungs or heart.

Clinton’s 2012 clot turned up during routine testing after a concussion she suffered in a fall after reportedly becoming severely dehydrated with an intestinal infection. Her concussion attracted widespread notice because it caused her to delay her scheduled December 2012 testimony before Congress about the attack on a U.S. diplomatic compound in Benghazi, Libya, three months earlier.

The clot in Clinton’s brain — known as a transverse sinus venous thrombosis — did not cause a stroke or neurological damage, according to her doctors. Like the other clots, it also resolved itself.

Information about the concussion and clot emerged in dribbles, leading to questions about Clinton’s unexplained absence from her job.

Despite the clot’s disappearance, Clinton’s doctors put her on Coumadin as a precaution. The drug’s generic form, warfarin, was first used as a rat poison before becoming the most widely used blood thinner in the United States.

The drug requires frequent testing to strike the right balance between preventing clots and the risk of excessive bleeding.

Bardack’s letter lists the blood clots and details Clinton’s struggle with concussion syndrome, including double vision.

Bardack writes that the only regular medications Clinton takes are Coumadin and a common thyroid hormone-replacement pill. The letter notes normal blood pressure, respiratory rate and cholesterol levels.

Scheiner, Obama’s former physician, has insisted that presidential candidates be compelled to provide a detailed medical picture to voters. In an opinion piece in Sunday’s Washington Post, he wrote that Trump, at 70, would be the oldest person to enter the Oval Office, while Clinton, at 68, would be a close second behind Ronald Reagan. And yet both have left lingering questions about the state of their health.

Scheiner said Clinton’s episode in New York is a reminder of how the status quo is lacking when it comes to how the country evaluates the health of potential leaders.

“We should have complete health reports on both of them. The whole process has to be changed,” Scheiner said. “We have to come up with something that’s an objective evaluation of their health.”

Brady Dennis contributed to this report.