He was thought to be suffering from a virus, but blood tests on Sunday evening showed he had contracted malaria while on international duty in Ghana at the African Cup of Nations last month.
"He looked terrible when we got up to Middlesbrough," Royals boss Steve Coppell told the Reading Evening Post. "The weather was bad and it was really windy. He was sweating really badly so we just thought it was flu.
"We flew him back on Saturday. He came into training on Sunday and felt better, but by that evening he felt worse and he went to hospital where blood tests showed he had malaria.
"He was taking malaria tablets out in Ghana so we're not sure how he got it. I'm not sure how long he'll be out of action for, but we're hoping it won't be much longer than a week."