Hillary Clinton is calling off plans to visit the West Coast on Monday and Tuesday following a health episode at the 9/11 memorial.

Clinton abruptly left the anniversary ceremony on Sunday morning and needed to be held up by three people before she appeared to stumble off a curb and was helped into a van.

The former secretary of state later emerged from her daughter's nearby apartment, saying she was "feeling great," but several hours later, her campaign revealed she was diagnosed with pneumonia on Friday and advised to rest.

She later called off plans to fly to California on Monday morning for two days of fundraising, campaign events and an appearance on Ellen DeGeneres's talk show.

The episode focused attention on the Democratic candidate's health with eight weeks remaining in a contentious election in which Republican Donald Trump has tried to sow doubt about her health and fitness to serve.

The billionaire businessman, who also attended Sunday's memorial, said, "I don't know anything," when asked about Clinton.

Trump has repeatedly questioned Clinton's health, telling supporters last month she "lacks the mental and physical stamina" to serve as president and fight Islamic State in Iraq and Syria group militants.

Hillary Clinton leaves her daughter's New York apartment after leaving early from 9/11 memorial ceremony 0:52

It's an accusation that Clinton has tried to play off as a "wacky strategy" from Trump and evidence he embraces an "alternative reality." She poked fun of the idea during an appearance on ABC's Jimmy Kimmel Live last month, jokingly opening a pickle jar as proof of her vigour.

Health of both candidates not fully known

Despite the intense focus on the Clinton's falling ill, Arthur Caplan, a bioethicist at New York University Langone Medical Center, said the moment told voters little about Clinton's physical fitness.

"There are plenty of people who may stumble around on a hot humid day for lots of reasons," Caplan said. "Without examining, without having the history ... you don't have a basis to say anything."

Clinton's campaign released a statement from her doctor, Lisa Bardack, that said Clinton was diagnosed with pneumonia on Friday.

"She was put on antibiotics, and advised to rest and modify her schedule," Bardack said in a statement. "While at this morning's event, she became overheated and dehydrated. I have just examined her and she is now rehydrated and recovering nicely."

Republican presidential nominee Donald Trump, who has repeatedly questioned Clinton's health, was also at the Sept. 11 event on Sunday. (Andrew Harnik/Associated Press)

In 2008, Republican nominee John McCain made public more than a thousand pages of his medical history to show he was cancer-free and fit to serve as president at age 71.

Neither Clinton nor Trump has released anything approaching that level of detail.

Bardack, an internist who has been Clinton's personal doctor since 2001, released a two-page letter in July 2015 that said Clinton was in "excellent physical condition and fit to serve as president of the United States."

Trump's gastroenterologist, Dr. Harold Bornstein, wrote a four-paragraph letter claiming Trump would be "the healthiest individual ever elected to the presidency." Bornstein later told NBC News it took him just five minutes to write it.

While Clinton has released more information than Trump, Caplan said neither candidate has offered voters a sufficient record. He said that ideally presidential nominees should allow an independent panel to assess their health.

"Since we can't get that done for taxes, I don't think we're going to get it done for health," he said, referencing Trump's refusal to match Clinton's release of her personal tax records.

Trump would be oldest 1st-term president

At 69, Ronald Reagan was the oldest person to be elected president when he won in 1980. Trump turned 70 in June, while Clinton will have just turned 69 if she wins the White House.

Reagan's fitness came under question in the 1984 election when he appeared fatigued and forgetful in a presidential debate with Democrat Walter Mondale. Reagan rebounded in subsequent debates and won in a landslide.

Hillary Clinton left the Sept. 11 commemoration ceremony early. Her doctor later said she had been diagnosed with pneumonia on Friday. (Justin Sullivan/Getty Images)

Aging researcher S. Jay Olshansky, of the University of Illinois at Chicago, said Sunday that age alone shouldn't be a disqualifier for presidential candidates. While people are increasingly vulnerable to illness as they age past 70, there are also better medical treatments than ever before. Our "concept of old," he stressed, has changed.

"I don't think age should be used at all," Olshansky said. "We shouldn't be judging people based on their age, but based on their ideas."

Clinton's health has been a lingering source of speculation among her critics, dating to well before she announced her second White House campaign. Republican strategist Karl Rove called a concussion Clinton sustained in 2012 a "serious health episode" and suggested two years later she may have suffered a brain injury.