What if Hillary Clinton's stumble leaving the 9/11 memorial service hadn't been caught on video?

Yes, there is little doubt that Clinton's abrupt departure from the event, the immediate quiet from her campaign and the shift in her campaign's statements from being "overheated" to having pneumonia would have been a news story.

Yet the images captured on video by a witness were what most catapulted Clinton's health from a topic that had been speculatively simmering on conservative web sites and among Republican nominee Donald Trump's supporters into a full-fledged media obsession in the 2016 campaign. Headlines about Clinton's health overran the news cycle on Sunday and Monday, brought to life by a video that appeared to capture a wobbly Clinton being helped into a waiting van.

"We have this visual moment, this powerful visual moment that people are seeing over and over and over again," said Julian Zelizer, a professor of history and public affairs at Princeton University. "Had the Secret Service protected her at that one moment - had they surrounded her more effectively - the story might not have had the same amount of impact. A matter of inches can make a huge difference in this day and age."

The episode prompted Zelizer and other presidential historians to recall the effects that other videos of ill candidates or sitting presidents have had on their campaigns or their presidencies. The days of Woodrow Wilson being able to hide a stroke or even John F. Kennedy to hide his lifelong health issues and extensive pharmaceutical regimen seem quaint in a media landscape dominated not only by the 24/7 cable news but social media that thrives on disseminating video online.

"We're in a whole other era, where there's so much space for the images to be circulated," Zelizer said. "It's triply damaging for the candidate."

Clinton faints as she leaves 9-11 memorial

When imagery is less present, says Rice University historian Douglas Brinkley, questions about a candidate's health can be less of an issue. John Kerry, for instance, was sidelined for several weeks during his presidential campaign to have surgery for prostate cancer, but it got less attention. "There's not a visual of him being operated on or stumbling into the operating room," Brinkley said.

Sometimes, candidates find ways to move past video images that call into question their health. Zelizer recalls Ronald Reagan's 1984 debate with Walter Mondale, when the performance of a rambling President Reagan raised questions about his mental capacity.

"He was stumbling over his words and seemed really confused," Zelizer said. "There were questions about whether he was too old and not-so-subtle questions about whether his memory was intact. Watching that had a very visceral effect."

Yet at the next debate, Reagan bounced back with his memorable line: "I will not make age an issue in this campaign. I will not exploit, for political purposes, my opponent's youth and inexperience." Reagan was later diagnosed with Alzheimer's, of course, but not before going on to win the 1984 campaign in a landslide, losing only one state.

Other video of presidents feeling ill become seared into our collective memories. George HW Bush famously got sick while suffering from gastroenteritis at a state dinner in Japan in early 1992, an election year.

"If we didn't have film footage of George W Bush throwing up on the Japanese Prime Minister's lap, it would have just been a back story," Brinkley said. "But the fact that we saw that footage allowed Pat Buchanan to start attacking Bush, and allowed him to think Bush was weakened... The visual was played over and over again by comedians. I can see it all in my mind right now."

Worse is when a video serves to reinforce a broader narrative that exists about a candidate. Brinkley cites Jimmy Carter, who went jogging in a challenging six-mile road race in 1979 and collapsed from heat exhaustion during the race. The race came two months after Carter gave what became known as his famous "malaise" speech, a popular speech Carter squandered by firing his Cabinet shortly thereafter, a move viewed as something of a government meltdown. "It was seen as the collapse of the Carter administration," Brinkley said. "You're getting footage that becomes symbolic."

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How Clinton's own wobble will ultimately be viewed by voters is unknown. After months of commentary about Clinton's health from conservative media, the video "plays into the alterna-right's narrative," Brinkley said. Yet her campaign says it will release even more medical details in the coming days and pneumonia is hardly an uncommon illness.

Meanwhile, just as video can raise questions about a candidate's health, it can also help to answer them. In the coming days, Clinton is sure to get in front of the cameras again, which should help allay the hysteria that's sprung up around her health. And according to a report Monday by Politico, Clinton quickly felt better and was ready to leave her daughter Chelsea's apartment in New York to go home.

"But she had to remain in place to wait for her travelling press pool - which had been penned in at ground zero - to arrive so they could witness her walk out to an SUV, under her own steam," the report said.