Footage has emerged of Democratic presidential candidate Hillary Clinton seemingly faint as she is carried into a van by her aides.

ANALYSIS: Hillary Clinton falling ill at a memorial service on the 15th anniversary of the September 11 attacks will catapult questions about her health from the ranks of conservative conspiracy theory to perhaps the central debate in the presidential race over the coming days.

"Secretary Clinton attended the September 11th Commemoration Ceremony [on Sunday, local time] for just an hour and thirty minutes this morning to pay her respects and greet some of the families of the fallen," spokesman Nick Merrill said.

"During the ceremony, she felt overheated, so departed to go to her daughter's apartment and is feeling much better."

REUTERS Democratic presidential nominee Hillary Clinton says she is feeling "great" after she became "overheated" at a September 11 memorial ceremony in New York City and departed the event early. Jillian Kitchener reports.

What that statement leaves out is that a) it came 90 minutes after Clinton left the ceremony b) reporters - or even a reporter - were not allowed to follow her and c) the temperature in New York City at the time of Clinton's overheating was in the 80s (between 26-29 degrees C). (A heat wave over the eastern United States broke Saturday night/Sunday morning.)

READ MORE:

* Clinton faints at 9/11 service

* 15th anniversary of attacks marked

* Clinton calls Trump supporters 'deplorables'

* Facebook co-founder pledges millions to Clinton

BRIAN SNYDER/REUTERS Hillary Clinton's smile later in the day won't be enough to deflect questions about her health.

She later left her daughter's apartment, saying she was "feeling great" and waving at the crowd, said the Associated Press.

Whether Clinton likes it or not, her "overheating" episode comes at a very bad time for her campaign. Thanks to the likes of Rudy Giuliani and a small but vocal element of the Republican base, talk of her health had been bubbling over the past week - triggered by a coughing episode she experienced during a Labor Day rally.

That talk was largely confined to Republicans convinced that Clinton has long been hiding some sort of serious illness. I wrote dismissively of that conspiracy theory last week, noting that Clinton had been given an entirely clean bill of health by her doctors after an episode in which she fainted, suffered a concussion and then was found to have a blood clot in late 2012 and early 2013.

BRIAN SNYDER/REUTERS Hillary Clinton climbs into her van outside her daughter Chelsea's home in New York, after resting following her "overheating" episode.

Coughing, I wrote, is simply not evidence enough of any sort of major illness that Clinton is assumed to be hiding. Neither, of course, is feeling "overheated." But those two things happening within six days of each other to a candidate who is 68 years old makes talk of Clinton's health no longer just the stuff of conspiracy theorists.

Whereas Clinton and her campaign could laugh off questions about her health before today, the "overheating" episode makes it almost impossible for them to do so. Not only has it come at a time when there was growing chatter - with very little evidence - that her health was a problem but it also happened at a 9/11 memorial event - an incredibly high-profile moment with lots and lots of cameras and reporters around.

Her campaign may well try to dismiss this story as nothing more than an isolated incident, meaning nothing. (Democrats were already pushing the story of George W. Bush fainting in 2002 after choking on a pretzel, via Twitter.)

BRIAN SNYDER/REUTERS The faint puts pressure on Clinton to address her health on the campaign trail - and likewise for Trump.

But the issue is that Clinton kept reporters totally in the dark for 90 minutes after her abrupt departure from the 9/11 memorial service for a health-related matter. No reporter was allowed to follow her. (Clinton has resisted a protective pool for coverage because Donald Trump refuses to participate in one.) This is, yet again, the Clinton campaign asking everyone to just trust it. She got overheated! But she's fine now!

Clinton may well be totally fine - and I certainly hope she is. But we are 58 days away from choosing the person who will lead the country for the next four years, and she is one of the two candidates with a real chance of winning. Taking the Clinton team's word for it on her health - in light of the episode on Sunday morning - is no longer enough. Reasonable people can - and will - have real questions about her health.

I wrote this on Tuesday morning:

"The simple fact is that there is zero evidence that anything is seriously wrong with Clinton. If suffering an occasional coughing fit is evidence of a major health problem, then 75 percent of the country must have that mystery illness. And I am one of them."

Well, that is no longer operative. Context matters. A coughing episode is almost always just a coughing episode. But when coupled with Clinton's "overheating" on Sunday morning - with temperatures something short of sweltering - Clinton and her team simply need to say something about what happened (and why the press was in the dark for so long.)

And as the New York Times' Adam Nagourney tweeted on Sunday morning, now might be a good time for Clinton to release a fuller record of her medical history:

"Feels like a good day for Clinton to release her medical records and call on Trump to do same"

Sunday morning changed the conversation in the race about Clinton's health. Or rather it will force Clinton to have a conversation about her health in the race.