United States democratic presidential nominee Hillary Clinton was diagnosed with pneumonia two days before she left a 9/11 memorial ceremony due to what her campaign claimed was overheating.
On Sunday, Ms Clinton said she was "feeling great", after a video showed Ms Clinton appearing to faint and stumble as she was led to a waiting van by her aides.
Her doctor Lisa Bardack has now revealed she was diagnosed with pneumonia on Friday.
"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," Dr Bardack, said in a statement.
"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."
After some US media reported she had fallen ill, reporters travelling with the candidate did not get an immediate response from the campaign to questions about her whereabouts and her health.
In a letter released by her doctor in July, Ms Clinton was described as being in "excellent health" and "fit to serve" in the White House.
However, Ms Clinton may not have time to rest. She is due to fly to California on Monday and Tuesday for a series of fundraisers.