What would happen if Hillary Clinton dropped out of US election race after being diagnosed with pneumonia?

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?

'Uncharted political territory'

Mrs 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 "unchartered 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 Mrs Clinton herself decided to withdraw. 

"We can make contingencies, argue, plead with Hillary Clinton, but DNC bylaws are clear her nominee status now totally up to her," Mr Shuster quoted the operative as saying. 

What happens if she drops out?

Were she to decide to step down, Joe Biden and Bernie Sanders 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 at a special meeting 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, Mrs 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 Mrs 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.  

Clinton's health ... and that cough

Mrs 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.

Mrs 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".

It's possible Mrs Clinton may have assumed that symptoms from an earlier viral infection were due to allergies, Dr William Schaffner, an infectious disease expert at Vanderbilt University, told the Associated Press.

Mrs Clinton takes antihistamines, which can "dry you out," and dehydration leads to a person being susceptible to heat exhaustion, Dr. Sharon Bergquist, an Emory University assistant professor of medicine who specialises in internal medicine, told AP.

Add in the possibility of fever, shortness of breath or other possible symptoms from pneumonia, and you have a constellation of factors that could have explained her feeling weak on Sunday, she said.

How serious is pneumonia?

Pneumonia is an infection of the lungs. Often it's a bacterial infection that sweeps in after a cold or flu virus.

In the UK, pneumonia affects around 8 in 1,000 adults each year, according to the NHS. It can affect people of any age, but it's more common – and can be more serious – in the very young or the elderly.

Dr Amesh Adalja, an infectious disease expert and senior associate at the UPMC Centre for Health Security in Pittsburgh who is not treating Mrs Clinton, said coughing is a cardinal symptom of pneumonia.

Recovery from pneumonia, the eighth leading cause of death in the United States, can be variable, he said, adding it takes a week for most patients to get better. 

History of health in campaigns

Speculation about a presidential candidate's health is nothing new.

Rumours about Franklin D Roosevelt’s health dogged his final presidential campaign in 1944. He continued to campaign energetically, though, and his aides managed to keep quiet the extent of the heart disease that would kill him the following year.

Since then, Ronald Reagan, Bob Dole and Senator John McCain all faced questions about their health as Republican presidential nominees. Mr McCain, the failed 2008 Republican presidential nominee, allowed reporters to see 1,173 pages of medical records after concerns were raised about a cancer scare.

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. "I think they did the right thing. 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, in December, Mr Trump's doctor wrote in a short letter that was made public that his blood pressure and laboratory results "were astonishingly excellent" and that he would be "the healthiest individual ever elected to the presidency."