But Sunday's drama was merely a capstone on Clinton's rough 48 hours.
Clinton aides spent Saturday cleaning up her remark that "half" of Trump's supporters were "deplorables," meaning racists, sexists and homophobes. The remark, for which she later expressed "regret," suddenly united a Republican Party that has struggled to get behind its divisive nominee.
Aides were scrambling to repair the damage Monday with Clinton's expected absence from the campaign trail for several days threatening to provide an opening for Trump to capture the initiative at a moment when he is closing in the polls.
While pneumonia is a relatively common complaint, the Democratic nominee, 68, now faces increasing political pressure to demonstrate she is fit for the considerable physical demands of the presidency. Her campaign is also confronting new questions over transparency after it kept details of her condition quiet for two days.
Trump expressed sympathy Monday when asked what he knew about Clinton's health in television interviews -- apparently seeking to avoid characteristically inflammatory remarks which would have made the story about him, rather than his rival.
"I really just don't know. I hope she gets well soon," Trump told Fox News, adding later, "Something is going on but I just hope she gets well and gets back on the trail, and we'll be seeing her at the debate."
But Trump, 70, who has made public fewer details about his health than Clinton has, could not resist raising veiled doubts about Clinton's true condition.
"They say pneumonia on Friday, but she was coughing very, very badly a week ago, and even before that, if you remember. This wasn't the first time. So, it's very interesting to see what is going on," Trump said on CNBC.
The billionaire also said during his Fox interview that he had a doctor's physical last week
and would release "the numbers" when he has them.
Advisers to Clinton are discussing how to move beyond arguably the worst weekend of the campaign. Any attempt at turning the corner on transparency -- after taking reporters' questions for four-straight days last week -- "has obviously been erased," one aide said.
Health issues are always difficult territory for presidential candidates, who are forced to cede privacy that regular people take for granted. But there will still be questions asked why Clinton, after days of speculation about her health, was not more forthcoming with her diagnosis.
The health of a potential President is considered of vital public knowledge given the intense pressures of the job and concentration of powers over life and death that flow through the Oval Office.
CNN contributor and former Barack Obama strategist David Axelrod encapsulated the campaign's problem when he tweeted
, "Antibiotics can take care of pneumonia. What's the cure for an unhealthy penchant for privacy that repeatedly creates unnecessary problems?"
Clinton campaign communications director acknowledged on Twitter that the campaign "could have done better," she used a pair of tweets Monday to put the focus back on their opponent.
"In contrast to HRC, Trump has been less transparent than any nominee in modern history," she tweeted
Clinton campaign officials pushed back on the suggestion that their delay in announcing the candidate had pneumonia amounted to intentionally misleading the public, but acknowledged that Clinton herself didn't fully convey how she felt.
Many people inside the Clinton campaign did not know about the pneumonia diagnosis on Friday. An aide told CNN that Clinton "thought she could push through this ... she was feeling better" -- until Sunday morning, when she went to a September 11 memorial event in Manhattan but was forced to leave early.
Brian Fallon, Clinton's national press secretary, told CNN that the campaign will release more medical information on the candidate later this week.
"The reality is, the pneumonia is the extent of what she has been diagnosed with. There are no other undisclosed conditions," Fallon told CNN's Wolf Blitzer. He added later, "We continue to want to be as transparent as possible on this, but at some point, people have to ask the same questions of Donald Trump"
The double weekend blows came at just the wrong time for Clinton, as Trump closes in the polls and pressure builds ahead of the first presidential debate in two weeks -- an event shaping up to be a potentially pivotal moment of the campaign.
Whether Clinton's rocky weekend will turn out to be just another unexpected twist in an election season that has had everything, or exert a lasting political impact will only become clear in the coming days.
She canceled a trip to California scheduled for Monday and Tuesday, though Clinton is expected to call in to Monday's evening fundraiser. The speed of her recovery and the way her enemies handle the episode will do much to shape how voters respond to her health issue.
Both Clinton and her husband former President Bill Clinton are known for punishing public schedules, despite health issues they have both had in the past, so it would not be a surprise to see the former secretary of state seek to get back on the campaign trail as soon as possible.
Hillary Clinton isn't the only member of her campaign who has been sick with respiratory issues. Some of Clinton's top aides -- including those who travel with her -- have been felled with respiratory issues in the last month, sources tell CNN. At least three senior aides have been ill, including at least one of whom was hospitalized and treated for dehydration.
Clinton's spokesman Nick Merrill said she was feeling better on Monday but was following her doctor's recommendation to stay home and rest. Aides said the Democratic nominee would work on debate prep for her debate clash with Trump -- and event that takes on even more significance in the light of Clinton's health episode.
Weekend on defense
But a weekend on defense and a possibly reduced schedule going forward threatens to slow Clinton's campaign at an unwelcome moment and will do little to calm increasingly jittery Democrats who only weeks ago were speculating about the possibility of an electoral landslide.
Donna Brazile, the interim chair of the Democratic National Committee, released a reassuring statement late Sunday wishing Clinton a "speedy recovery."
"I look forward to seeing her back out on the campaign trail and continuing on the path to victory," she said.
It was bad enough for Clinton that she had to leave the ceremony marking the 15th anniversary of the September 11 attacks early -- setting off alarm bells among her traveling press pool. But the later emergence of video showing her wobbly, staggering and stumbling before being helped into her black van conjured up the kind of image, played over and over on television, that campaign strategists dread.
The footage was more than a blow to her dignity. It will be used by opponents to validate a months-long campaign of rumors and innuendo about the true state of Clinton's health.
Trump has frequently cast doubt on Clinton's physical fitness, saying she lacks "stamina" and takes naps in the afternoon and runs on an easy schedule -- a claim debunked by reporters who follow her campaign.
Aware of potential impact
The Clinton campaign was clearly aware of the potential impact of the video. After resting at her daughter Chelsea's apartment, the Democratic nominee emerged smiling, and under her own power in front of the cameras, taking a picture with a young girl before climbing into her motorcade.
She told reporters she was "feeling great" and parried further questions by commenting that it was a "beautiful day in New York."
The campaign also tried to foster an air of normality by saying that Clinton spent time playing with her grandchildren while at her daughter's home.
But hours of speculation and uncertainty about what happened to Clinton and a lack of information about her status triggered an air of crisis.
When details about her condition finally emerged -- in the form of a statement issued through the campaign by Clinton's physician Lisa Bardack -- they only added to impressions that the campaign abhors transparency.
Advisers to Clinton are also discussing Monday how to move beyond arguably the worst weekend of the campaign. Any attempt at turning the corner on transparency -- after taking reporters' questions for four-straight days last week -- "has obviously been erased," one aide said. Clinton campaign officials pushed back on the suggestion that they were intentionally misleading the public, but acknowledged that Clinton herself didn't fully convey how she felt.
Given that Trump is 70 and would be the oldest person to take the oath of office as President for a first term, and Clinton would be the second oldest, both candidates are certain to face pressure for a more comprehensive accounting of their health.
Bardack issued a letter late last year saying Clinton was in good health and fit to serve as President. Trump has offered only a bizarre report from New York physician Harold Bornstein, saying that he would be the "healthiest individual ever elected President."
Clinton is likely to resume campaigning Wednesday, but will almost certainly not fly to Nevada. She is more likely to give her economic-themed speech in a battleground state far closer to New York: Pennsylvania, New Hampshire and Ohio are possibilities.
Difficult territory for candidates
Ever since Clinton sustained a blood clot and a concussion after a fainting episode near the end of her tenure as secretary of state in 2012, she has faced a swirl of conspiracy theories about her health.
She joked about the criticism on ABC's "Jimmy Kimmel Live" last month.
"Back in October, the National Enquirer said I would be dead in six months. So with every breath I take, I feel like I have a new lease on life," Clinton said.
If Clinton is forced to take time off, it will fall to her high-profile surrogates to pick up the slack. The President, for instance, is due to campaign for his former rival and secretary of state Tuesday in Philadelphia. His wife, Michelle Obama, will hit the campaign trail as well next week.
Clinton had hoped for better headlines after the first frenzied post-Labor Day week on the trail.
After months of dodging the press, she invited reporters to travel on her new campaign jet last week and took questions. On Friday, she gathered a group of high ranking former national security officials and military brass and delivered a presidential-style statement to stress her suitability to lead national security policy in the Oval Office.
Polls, however, show that Clinton's lead has dramatically narrowed in recent weeks, amid unflattering coverage of the controversy over her email server and the Clinton Foundation.
A CNN/ORC poll last week had behind Trump by 2% among likely voters and there are also signs of tightening in swing state polls -- despite Clinton still having many more routes to 270 electoral votes than Trump
But in one ray of sunshine for Clinton on Sunday, an ABC News/Washington Post poll showed her up 5% on Trump among likely voters.