Mrs Hillary Clinton's health and her stamina to run the country is in the spotlight as the Democratic nominee is treated for pneumonia and dehydration after abruptly leaving a Sept 11 memorial ceremony in New York.
With two months of the gruelling United States presidential campaign to run, and national polls showing her lead over Republican rival Donald Trump shrinking, Sunday's incident is likely to intensify calls for the 68-year-old and Mr Trump, 70, to release more detailed medical records - information both have seemed unwilling to share.
"Secretary Clinton has been experiencing a cough related to allergies. On Friday, during follow-up evaluation of her prolonged cough, she was diagnosed with pneumonia," Mrs Clinton's doctor Lisa Bardack said in a statement. "She was put on antibiotics and advised to rest and modify her schedule."
Her campaign, however, had not released any information on the pneumonia diagnosis before the incident, again raising questions of transparency.
"While at this morning's event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely," said Dr Bardack.
How pneumonia affects people
Q What is pneumonia?
A Pneumonia is a common lung infection that affects millions of people each year. It is most often caused by bacteria, though viruses account for up to one-third of the cases in the United States each year, according to the National Heart, Lung and Blood Institute. Fungi and other germs can also cause pneumonia.
When germs reach the lungs, the immune system responds by sending cells to attack them. Those cells can cause air sacs in the lungs to become inflamed or filled with fluid and pus.
Q Who is at risk?
A While it is an infection that most healthy people overcome, it can be more serious for people aged 65 or older, according to the institute. Mrs Clinton is 68. It also tends to be more serious for infants and young children.
Q What are the symptoms?
A The severity of symptoms depends on the type of pneumonia a person has, as well as one's age and health. Typical symptoms include coughing, fever, chills and shortness of breath, according to the American Lung Association. There could also be other symptoms such as excessive sweating, loss of appetite and low energy. Older people sometimes experience confusion.
Bacterial pneumonia can drive the body's temperature to as high as 40 deg C.
With viral pneumonia, symptoms are typically more similar to influenza: fever, dry cough, headache or muscle pain.
Q How does recovery occur?
A Bacterial pneumonia can be treated with antibiotics, with most people improving within one to three days. Antibiotics do not fight viral infections, but doctors may prescribe them for viral pneumonia to prevent further infection. Drinking fluids and resting can help.
Mrs Clinton's doctor did not identify the type of pneumonia she has, but said she was prescribed antibiotics and advised to rest.
It can take as little as a week for a healthy young person to feel normal again, but it could be weeks before older people regain their full strength. NYTimes
However, on Sunday night, Clinton campaign spokesman Nick Merrill said she had cancelled travel plans to California for yesterday and today, where she was supposed to attend fund-raising events. There was no word on whether she would go ahead with plans to campaign in the Las Vegas area tomorrow.
Adopting a respectful tone when he spoke about the incident, Mr Trump said in a Monday morning interview with Fox News: "Something is going on, but I just hope she gets well and gets back on the trail, and we will be seeing her at the debate." He added that he would be releasing detailed health information in the coming days.
A video posted on Twitter showed Mrs Clinton's legs buckling as she walked towards her van on Sunday. Her feet also dragged as two Secret Service agents hoisted her into the vehicle, which whisked her off to her daughter's apartment in Manhattan.
About 90 minutes later, she emerged from the apartment, waving to onlookers and telling them that she felt much better. She also hugged and took a photo with a little girl who approached her.
The incident follows a coughing fit at a Labour Day rally last week, where Mrs Clinton joked that she was "allergic" to the thought of Mr Trump, and struggled to suppress her cough throughout the speech.
In the past, Mr Trump had questioned Mrs Clinton's health, saying she "lacks the mental and physical stamina to take on ISIS, and all of the many adversaries we face".
Critics have also cited a concussion she suffered in 2012, which left her with double vision and a blood clot, as evidence of her poor health.
A two-page doctor's letter, released in July last year, stated that Mrs Clinton was in "excellent physical condition and fit to serve as president of the United States".
In contrast, Mr Trump's letter of health issued by his doctor Harold Bornstein in December was only four paragraphs long and stated that he had "no significant medical problems" over the past 39 years. It also stated that Mr Trump, if elected, would be "the healthiest individual ever elected to the presidency".