The shocking images of the Democratic nominee being carried to a vehicle in a state of near-collapse six days ago have sent tremors through the presidential race. Not so long ago, Hillary Clinton had seemed on course for an easy win - perhaps even a landslide bigger than President Barack Obama's commanding victory in 2008. Back in early August, she was almost eight points clear of her Republican opponent, Donald Trump, in national polls.

Even before her health scare, her lead had shrunk to about three points. Now, the nerves of Trump opponents have been frayed by reputable polls putting him in the lead in several of the battleground states that will decide the election's outcome. On Wednesday, two polls from the perennial swing state of Ohio gave Trump a five-point advantage.

Meanwhile, the Clinton campaign has been furiously trying to answer the most basic question of all: "Is she okay?"

That answer - assuming that one thinks her aides and her doctor are telling the truth - is, basically, yes. Clinton's swoon at an event in New York commemorating the September 11, 2001, terrorist attacks was caused by pneumonia, according to her doctor. The pneumonia had in fact been diagnosed two days before that, but had been kept secret from the public. Clinton took time off the trail until Thursday to recover, as high-profile surrogates including Obama campaigned on her behalf.

Trump initially declined to make any substantive comments about Clinton's health beyond wishing her well - an unusual display of self-restraint from a candidate who has often assailed his opponents in colourful terms, especially on Twitter.

The extent to which Trump held himself back was less-than-complete, however. At a Wednesday evening rally in Ohio, he referred to the heat of the venue.

"You think Hillary would be able to stand up here for an hour and do this?" he asked. "I don't know."

There are other difficulties for Clinton. One is the simple fact that her stumble in New York was captured on video by a bystander with a phone. The imagery was far more disturbing than a statement from her campaign acknowledging she was ill would have been.

Millions of Americans saw Clinton's body go limp, her head appear to loll and, finally, what looked like a complete collapse as she was lifted by secret service agents into a waiting van.

When it comes to bad news, it's always worse when there are pictures. Past presidents found this out to their cost, too.

The first President George Bush became the butt of a thousand jokes when he vomited during a state dinner in Tokyo hosted by the Japanese prime minister in early 1992. Later that year, Bush lost his battle for re-election, to Bill Clinton.

In 1979, President Jimmy Carter ill-advisedly over-exerted himself while running a six-mile race. The photos of him on the brink of collapse were disastrous - even though he was suffering only heatstroke and not, as some people feared, a heart attack. Carter, too, lost his re-election fight, to Ronald Reagan.

There is another, additional complication for Clinton. Her health has been the subject of feverish speculation from conservatives, especially in the wilder corners of the internet.

The rumour mill first sprang into action in December 2012. Clinton fell at her home, suffering a concussion. Although the concussion was initially said to be minor - Clinton had fainted because of a stomach virus - events took a more serious turn around New Year's Day 2013, when doctors detected a blood clot in a vein in her head.

Clinton was hospitalised for several days, though the treatment was nothing more invasive than the use of blood thinners.

The episode launched various conspiracy theories that Clinton was actually suffering from a more serious illness. Brain cancer, multiple sclerosis and Parkinson's disease were all mentioned - without any evidence to back up such allegations. Those theories had been given a bigger push into the mainstream more recently, just before Clinton's bout of pneumonia.

Trump had archly suggested several times that she lacked the "stamina" to be president.

One of his highest-profile supporters, former New York City Mayor Rudy Giuliani, last month asserted that Clinton was exhibiting "several signs of illness". Upon being challenged by the presenter of the Fox News show on which he made that charge, Giuliani responded: "So go online and put down 'Hillary Clinton illness' - take a look at the videos for yourself."

While Clinton remained healthy, those innuendoes were easily brushed aside by her supporters.

There was even a possibility that the comments from Giuliani, as well as a number of similar suggestions from prominent conservative media personalities, would backfire, seeming just too wacky and conspiratorial.

Now that danger has all but disappeared. To the enormous frustration of the Clinton campaign, the onus is on them to prove she is suffering from nothing worse than pneumonia. The letter from Clinton's physician attesting to the candidate's overall good health is about as much as they can do, but it won't be enough for some.

Amid all the shock and speculation, it's worth emphasising that Clinton remains the favourite to win the race for the White House.

Her current polling decline may well be reversed as memories of the New York incident fade and she proves her vigour on the campaign trail. Her campaign's ability to get out the vote in an organised fashion is superior to Trump's, in the opinion of most experts.

And, for all of Clinton's anaemic poll ratings, Trump's are even worse. Trump is the most unpopular major-party presidential nominee of modern times. That factor alone would doom him to defeat in a normal election year.

But this election has been anything but normal. And there are things that could yet go wrong for Clinton, beyond the issue of her health.

The presidential debates are just around the corner, with the first of three clashes set for September 26 at a university just outside New York City.

Clinton is an accomplished debater but a single gaffe could overshadow everything. Meanwhile, Trump could benefit from low expectations. Clinton backers fear he could earn media praise merely by avoiding disaster.

Among the other things giving Democrats bouts of sleeplessness: WikiLeaks founder Julian Assange, who has made no secret of his distaste for Clinton, has said that his organisation will reveal "significant" information closer to Election Day; any terrorist attack in the United States could affect the White House race in unpredictable ways; and, perhaps most plausibly of all, turnout could be a challenge for Clinton, in spite of her campaign's vaunted organisational strength.

Several recent polls have shown that Americans who intend to back her on November 8 are, as a group, less enthusiastic than would-be Trump voters.

But, above all, the sheer drama of what happened last weekend means that any further health problems for Clinton would be disastrous.

One more stumble would put her path to the Oval Office in grave danger.

Niall Stanage is associate editor of The Hill

Belfast Telegraph