Who will step in to the hot seat if Hillary drops out?
Despite assurances from Hillary Clinton's doctor that she was "recovering nicely", the scare raised concerns about her health and prompted questions about why she had failed to disclose the information, having been diagnosed on Friday.
With a trip to California this week cancelled and aides struggling to spin a positive angle from the development, the twist in the election campaign threw up an unwelcome question for the Democrats: What would happen if she had to drop out of the race due to ill health?
Ms Clinton can only be replaced as the nominee if she herself decides to step down - and there is no suggestion of that at this stage.
However, David Shuster, a journalist who has anchored for Al Jazeera America and MSNBC, quoted a Democratic operative as saying they had entered "uncharted political territory".
The unidentified source said the party may hold an emergency meeting to consider a possible replacement, but stressed selecting a new nominee would only arise if Ms Clinton herself decided to withdraw. "We can make contingencies, argue, plead with Hillary Clinton, but DNC bylaws are clear her nominee status [is] now totally up to her," Mr Shuster quoted the operative as saying.
Were she to decide to step down, Bernie Sanders and Joe Biden and would be the likely names linked to the vacancy. According to the rules of the Democratic Party, the national committee is responsible for filling the position. Party officials would each cast one vote and the winner is whoever gets the majority.
No special consideration would be given to Tim Kaine, the current vice-presidential candidate, or Bernie Sanders, Ms Clinton's closest rival in the race to secure the Democratic nomination. And if someone other than Mr Kaine were selected, he would remain the vice-presidential candidate.
When the unlikely possibility of Ms Clinton dropping out was raised months ago amid the furore surrounding her emails and whether she would be indicted, Mr Biden was widely considered the likely replacement at the time.
Ms Clinton has been in the news before for serious health issues. In December 2012, she suffered a concussion and shortly afterward developed a blood clot. In a letter released by her doctor in July 2015, Clinton was described as being in "excellent health" and "fit to serve" in the White House. It noted that her current medical conditions include hyperthyroidism and seasonal pollen allergies.
Ms Clinton had a coughing fit while campaigning in Cleveland early last week. It was dismissed by her campaign as allergies and by the Democratic nominee herself at the time as coming from "talking so much".
Speculation about a presidential candidate's health is nothing new.
Rumours about Franklin D Roosevelt's health dogged his final campaign in 1944. Since then, Ronald Reagan, Bob Dole and Senator John McCain all faced questions about their health as presidential nominees.
Political strategists said the Clinton campaign should confront the health issue head on to tamp down any concerns. Bud Jackson, a Virginia-based Democratic strategist, said the statement from the doctor was a good start. He said the incident should encourage more transparency from the campaign about her health. "They had her examined and put out a statement. It means less speculation," he said.
Donald Trump has also faced calls to release detailed information on his health and medical history. Instead, Mr Trump's doctor wrote in a short letter that he would be "the healthiest individual ever elected to the presidency."