U.S. Democratic presidential candidate Hillary Clinton is being treated for pneumonia, her doctor said on Sunday, after the politician was taken ill at a ceremony marking the 15th anniversary of the 9/11 terror attacks. Clinton abruptly left the ceremony in New York on Sunday after becoming "overheated," one of her campaign spokesperson said following reports that she had "fainted." There was a further update later in the evening when her doctor released a statement saying Clinton had pneumonia. "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. She was put on antibiotics, and advised to rest and modify her schedule. 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," Dr Lisa R. Bardack said in a statement. Clinton was due to travel to California on Monday for a two-day fundraising and campaign trip, her spokesman Nick Merrill said, but continues to rest at home in Chappaqua, near New York. There have been questions over why Clinton didn't disclose her health condition earlier but her aides said the former Secretary of State didn't disclose her pneumonia because she thought she could power through and wasn't coughing Saturday. Now aides realize that was a mistake, NBC News reported.



Democratic presidential nominee Hillary Clinton sips water during a Labor Day rally September 5, 2016 in Cleveland, Ohio. Brendan Smialowski | AFP | Getty Images

It's unclear whether Clinton will make scheduled trips to Las Vegas on Wednesday and Washington DC on Thursday. With the U.S. presidential election just two months away, there are concerns that the condition is a late setback for Clinton at a crucial campaigning time. Clinton is scheduled to face her Republican rival, Donald Trump, in a presidential debate on September 26.

Her Republican rivals have already questioned Clinton's physical fitness and last month, Trump said she lacked "mental and physical stamina" to serve as president.Here's a quick Q & A on pneumonia and how long it could put Clinton out of action:

Q. What is pneumonia?

A. Pneumonia is inflammation of the tissue in one or both lungs which is usually caused by a bacterial infection. The alveoli (air sacs at the end of the breathing tubes in your lungs) can become inflamed and fill with fluid.

Q. What are the symptoms?

A. Symptoms of pneumonia include a dry cough, difficulty breathing, a rapid heartbeat, fever, chest pain, loss of appetite and generally feeling unwell – all symptoms that can be easily mistaken for other conditions such as the common cold, making diagnosing pneumonia sometimes difficult.

Q. What causes pneumonia?

A. Pneumonia can be caused by viruses, bacteria and funghi, the U.S. Centers for Disease Control and Prevention states. "In the U.S., common causes of viral pneumonia are influenza and respiratory syncytial virus (RSV), and a common cause of bacterial pneumonia is Streptococcus pneumoniae (pneumococcus). Pneumonia can also be caused as a result of being on a ventilator, known as ventilator-associated pneumonia," according to the CDC's website.

Q. Who's at risk?

A. Pneumonia can strike healthy people of all ages but there are certain groups of people at increased risk of developing the infection. These include babies and very young children, elderly people, those that smoke and people with other health conditions, such as asthma or a heart, kidney or liver condition or those with a weakened immune system, the U.K.'s National Health Service (NHS) website states.

Most cases of pneumonia are bacterial and aren't passed from one person to another but good hygiene is essential to stop viral strains of the condition. In the U.S., over 53,000 people died from pneumonia in 2013, according to the latest available data from the CDC. In the U.S. in 2014, 61.3 percent of adults 65 years of age or older were estimated to have received a pneumococcal vaccination, however.

Q. How bad is it?