The former secretary of state has been experiencing a cough related to allergies, her doctor Lisa Bardack said in a statement.

During a follow-up evaluation of her prolonged cough on Friday, "she was diagnosed with pneumonia. She was put on antibiotics, and advised to rest and modify her schedule," Bardack said.

Clinton, 68, became "overheated and dehydrated" at the ceremony marking the 15th anniversary of the 9/11 attacks at the Ground zero memorial in lower Manhattan early yesterday, the doctor said, adding that she examined Clinton and "she is now rehydrated and recovering nicely".

In December 2013, the former first lady had to be rushed to New York's Presbyterian Hospital after a medical scare following a fainting spell and concussion.

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