Pope Francis is being treated for a "complex clinical situation" and will remain in hospital for as long as necessary, the Vatican has said.
The 88-year-old was admitted to Rome's Gemelli hospital on Friday to undergo treatment and tests for bronchitis.
In an update on Monday, the Vatican has said the pontiff has a "polymicrobial infection" of his respiratory tract, which has required a change in his treatment.
Before his admission last week, the Pope had bronchitis symptoms for several days and had delegated officials to read prepared speeches at events.
Over the weekend, the Vatican said the Pope's condition was stable and that he'd been told to have "complete rest" to aid his recovery.
He was unable to deliver his regular weekly prayer on Sunday in St Peter's Square or lead a special mass for artists to mark the Catholic Church's Jubilee Year.
