Sergio Alfieri, the chief of the pontiff's medical team, said the 88-year-old was affected by a seasonal flu before being taken to hospital.
In a brief update on his health on Friday, the Vatican said Francis had had a good night, had got up and eaten breakfast.
On Thursday, the Vatican said the Pope's overall condition was "improving slightly" and his heart is working well.
Mr Alfieri said he was a "fragile patient" but was in "good spirits" and joking with medical staff.
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