The diagnosis is the latest in a long line of health problems the Pope has suffered in recent years.
On Sunday, Matteo Bruni, the Vatican spokesman, said the pontiff was “in good spirits” and had had a tranquil night before eating breakfast and reading the newspapers.
He did not indicate whether the Pope was suffering from a bacterial or viral infection.
The Pope’s doctors had earlier ordered complete rest. He was unable to deliver his regular weekly prayer to pilgrims in St Peter’s Square on Sunday or lead a scheduled mass for artists to mark the Catholic Church’s Jubilee Year.
The Vatican cancelled the Pope’s visit to Rome’s iconic Cinecitta film studios on Monday, and scrapped his regular weekly audience scheduled for Wednesday.
“I believe it is for a form of acute exacerbation of chronic bronchitis or pneumonia, which needs antibiotic treatment of seven to 10 days,” said Matteo Bassetti, director of infectious diseases at Genoa’s San Martino Policlinico Hospital.
“If the micro-organisms are particularly challenging, it’s going to be done intravenously, and then his hospitalisation would have to be prolonged as well,” he told Adnkronos, the Italian news agency.
Despite his condition, the Pope continued his recent practice of telephoning members of a Catholic parish in Gaza over the weekend.
One of the parishioners said the Pope had called on both Friday and Saturday to express his closeness to the people of Gaza.
“He was in good spirits, he sounded a bit tired but he wanted to know how we are doing,” he told Mediaset, the Italian broadcaster.
The Pope endured severe pneumonia and had part of a lung removed when he was younger.
He has suffered two falls in the past few months and now frequently uses a wheelchair at public events due to his weakening knees.
In 2021, he had surgery to remove part of his colon. He spent three nights at Gemelli Hospital in March 2023 for treatment of another bout of bronchitis.
“Thank you for the affection, prayer and closeness with which you are accompanying me in these days,” the Pope wrote on X on Sunday.