Clinton's doctor reveals she has a lung infection.

Hillary Clinton's doctor has revealed the 68-year-old has pneumonia - hours after video showed her collapsing into the arms of her Secret Service agents after being rushed from the 9/11 memorial service at the World Trade Center.

Dr. Lisa R. Bardack released a statement through her campaign team after she performed a check-up on the presidential candidate at her home in Chappaqua, New York, on Sunday, after the 'medical episode'.

She said: '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.'

Clinton is now holed up in her upstate New York home after a bystander recorded her losing her balance, slumping at the knees and being held up by her aides while waiting to get in a minivan.