The pontiff confirmed he was feeling unwell as he was forced to sit down during his Christmas speech.
Francis, 79, told the gathered he was "not feeling strong" as he took his seat.
It reignited concerns for the pope who fell twice in November.
In October, the Vatican was forced to deny reports Francis had been treated for a brain tumour.
Vatican spokesman Fr Thomas Rosica said: "There is a flu bug going around in Rome and elsewhere. But there is nothing to worry about."