WASHINGTON — Hillary Clinton has had pneumonia since at least Friday, her doctor said Sunday evening in response to questions about why she abruptly left a morning event commemorating the Sept. 11 attacks and appeared close to collapsing.

Clinton, 68, quickly recovered from what her physician, in a statement released by her campaign, called related dehydration. But the episode at the New York City event played into the hands of rival Donald Trump and other Republicans who have raised questions about her health and stamina.

It also highlights the lack of disclosure of detailed medical information about both candidates, who are among the oldest to ever seek the presidency. Trump, 70, has disclosed even less medical information than Clinton.

A spokesman for Clinton initially said that she was “overheated’’ at the outdoor event Sunday morning, where temperatures were in the low 80s but humidity was high. The presidential candidate was taken to her daughter Chelsea’s nearby apartment to recover, spokesman Nick Merrill said in a statement.

Shortly before noon, Clinton emerged from the apartment, appearing refreshed and well. She paused to take photos and waved to onlookers before heading to her Chappaqua home.

‘‘It’s a beautiful day in New York,’’ Clinton said. “I’m feeling great.’’

Later in the day, Clinton’s doctor released a statement revealing that the candidate was being treated for pneumonia, a common but serious respiratory illness.

“She was put on antibiotics, and advised to rest and modify her schedule. While at this morning’s event, she became overheated and dehydrated,’’ Dr. Lisa Bardack wrote. “I have just examined her and she is now re-hydrated and recovering nicely.’’

The campaign also announced that Clinton canceled plans to fly to California on Monday for two days of fund-raising, a public event, and an appearance on Ellen DeGeneres’s talk show.

Clinton’s illness opened a new conversation about her health as she enters the final and busiest stretch of campaigning before the Nov. 8 election. Clinton has not released any formal statements about her health since July 2015, when the same doctor issued a two-page letter.

Some specialists cautioned against making too much of Sunday’s episode.

“At present, we have no way of knowing whether either Mr. Trump or Secretary Clinton is in good health, because neither have released meaningful records, and to use this particular incident to suggest that one candidate is less healthy than the other may be effective politics, but as science, it is truly meaningless,’’ Dr. Jacob Appel, an assistant professor at Mount Sinai School of Medicine who has studied presidential medical history, wrote in an e-mail to STAT, the online publication covering health, medicine, and science. He called it “akin to predicting an influenza epidemic based [on] a candidate’s sneezing.’’

Compounding the problem for Clinton — at least from a public relations perspective — was that her moment of physical weakness Sunday was captured on a video from a bystander who posted it on Twitter.

The video shows Clinton being steadied by one aide as she waited for a black van to pull up to a curb. Then it shows her stumbling as she was assisted by two people who ultimately scooped her into the vehicle.

Trump attended the same morning event and was photographed standing just a few feet away from Clinton at times.

Clinton was already under pressure to offer additional details about her physical fitness, most recently after a lengthy coughing fit interrupted a speech she was delivering in Cleveland last week.

Clinton tried to make light of that event.

“I’ve been talking so much,’’ she said, struggling to catch her breath during the Ohio speech. “Every time I think about Trump, I get allergic.’’

She later assured reporters who travel with her that she had increased her antihistamine dosage, but said nothing about a more serious illness.

Clinton has had similar coughing episodes, including in February, when a coughing attack interrupted a speech she was giving at the Schomburg Center for Research in Black Culture in New York.

As with other problems that have dogged Clinton on the campaign trail, opponents said the candidate’s refusal to provide basic information, in this case health records, seeds distrust and skepticism about an issue that could have an easy explanation.

Bardack, Clinton’s doctor, wrote last year that Clinton was “in excellent physical condition’’ and “fit to serve as president of the United States.’’

In the letter, Bardack noted that Clinton has a common thyroid disorder and suffers from seasonal allergies. She said Clinton “exercises regularly’’ including yoga, swimming, walking, and weight training.

The most serious health issue Clinton has had, as far as the public knows, came in December 2012, toward the end of her tenure as secretary of state. It started with a stomach virus, then she became dehydrated, fell, and had a concussion, according to her doctor.

As she was being treated for the head injury, Clinton’s doctors discovered that she had a blood clot. She remains on blood-thinning medicine “as a precaution’’ even though her doctor found that she has no clotting disorders.

Trump has tried to make an issue of Clinton’s health, saying that she “lacks the mental and physical stamina’’ to occupy the Oval Office.

He declined to say anything Sunday, keeping a pledge not to campaign on the 15th anniversary of the Sept. 11, 2001, terror attacks.

Like Clinton, he has declined to make detailed health records public. Last year Trump released a letter from his doctor that contained even less information than Clinton’s and was widely mocked for making unsubstantiated claims.

“If elected, Mr. Trump, I can state unequivocally, will be the healthiest individual ever elected to the presidency,’’ according to the letter from his physician, Harold Bornstein. The doctor characterized Trump’s blood pressure as “astonishingly excellent’’ and described Trump’s physical strength and stamina as “extraordinary.’’

He later revealed in an interview with The Washington Post that it was written in five minutes as a limousine sent by Trump’s campaign staff waited outside his office to ferry it back to the campaign.

This week, Trump is pushing the issue into the TV talk show realm, with an appearance on “The Dr. Oz Show,’’ in which he is scheduled to discuss his “personal health regimen.’’

Material from the Associated Press was included in this report. Annie Linskey can be reached at annie.linskey@ globe.com.