US Democratic presidential nominee Hillary Clinton has a non-contagious, bacterial form of pneumonia and her campaign will release a doctor's letter on her treatment .

The letter, which will detail the treatment she received in the days before and after her pneumonia diagnosis, including a chest scan, says she was given a 10-day course of antibiotics to treat the condition.

Clinton has been evaluated several times by her physician, Dr Lisa Bardack.

"The remainder of her complete physical exam was normal and she is in excellent mental condition," Bardack's letter will state.

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She remains "healthy and fit to serve as President of the United States".

Additional health records released with the letter will show that Clinton's cholesterol and blood pressure are within normal ranges.

- Reuters, AAP, AP