A recent coughing fit, a fainting episode and an eventual diagnosis for pneumonia have all added to uncertainty concerning the former Secretary of State's future as the Democratic Party candidate for president. But what would happen if Hillary Clinton actually left the race?
What do the experts say?
Associate professor in US politics at Warwick University Dr Trevor McCrisken says the field would be open after what he believes would be the unlikely event of Clinton withdrawing from the race.
He told the Press Association: "It would really come down to the party. There is certainly no time to have another primary season."
BBC political presenter Andrew Neil tweeted: "If Mrs Clinton forced to drop out of presidential race, her replacement is entirely matter for the 445 strong Democratic National Committee.
The DNC rules give no guidance as to who would have preference if Mrs Clinton drops out. Sanders, Biden, Kaine would all be in running.
-- Andrew Neil (@afneil) September 12, 2016
"The DNC rules give no guidance as to who would have preference if Mrs Clinton drops out. Sanders, Biden, Kaine would all be in the running."
Does Clinton have an automatic successor?
There is no automatic successor to replace Hillary Clinton as Democratic Party presidential nominee if ill health forces her out of the race for the White House.
Experts believe the party's governing body, the Democratic National Committee (DNC), would move quickly to choose its new standard bearer in the November election should Clinton decide to quit.
Who could replace her, should she quit?
Despite left-wing candidate Bernie Sanders running Clinton hard in the primary selection elections, he would have no automatic right to replace her at the top of the party ticket.
Vice presidential candidate Tim Kaine would remain in his position if another person were chosen to succeed her.
Others in the running for the top spot if Clinton pulls out would include outgoing US vice president Joe Biden who considered challenging the former secretary of state for the Democratic Party crown in the primaries.
Does this give Trump an upper hand?
McCrisken said the pneumonia incident is likely to put more pressure on Republican Party challenger Donald Trump to release more information about the state of his own health.
"The pneumonia diagnosis could be spun to suit both sides of the campaign - by Trump to say that Clinton is weak, and by Clinton to show that she's resilient by continuing to campaign despite illness.
"However, appearing ill can affect popularity. At the first ever TV debate between Nixon and Kennedy in the US, Nixon had the flu and looked bad. Those listening on the radio said Nixon fared better; the TV audiences preferred Kennedy."
What do the bookies say?
Clinton's health scare has seen the odds on Mr Trump becoming the next US president tighten.
Betway now put a Trump victory at 11/8, while odds on Mrs Clinton securing the White House have lengthened from 2/5 to 8/15.
Betway's Alan Alger said: "Hillary's illness couldn't come at a worse time as the race had narrowed significantly in the past fortnight, with Trump gaining three or four points in the polls.
"Now presented with a 48-hour window without his rival on the campaign trail, Trump could make further inroads as we approach the all-important debates.
"However, Hillary is still our odds-on favourite to be the next President and Trump will need an impressive, composed performance during the debates to convince undecided voters in key swing states to give him their vote."