The Vatican said that the 86-year-old Roman Catholic pontiff would have to stay for "a few days" at Rome's Gemelli hospital for "appropriate" medical treatment.
Pope Francis spent a calm night in hospital after being admitted for a respiratory infection and is expected to remain there for a few days for treatment, a Vatican source said Thursday.
"The pope spent a good night" at Rome's Gemelli hospital, the source said, adding that there would be an update on the 86-year-old pontiff's condition later.
Pope Francis, who earlier this month marked 10 years as head of the Catholic Church, has suffered a number of health issues in recent years but normally has a busy schedule and continues to travel widely.
His admission to hospital on Wednesday, for what the Vatican initially said were pre-planned tests, forced him to cancel a number of engagements.
It also raises questions over his presence at services for the upcoming Holy Week and Easter, Christianity's most important holiday.
But the Italian news agency ANSA reported that nursing staff "are very optimistic" that, barring surprises, the pope could be discharged in time for Palm Sunday celebrations on April 2.
The Vatican late on Wednesday said the pope had a respiratory infection that would require hospital treatment for "a few days", adding that Covid had been excluded.
Over the past year Francis has suffered from chronic knee pain that has required him to use a wheelchair.
His postponement last year of a scheduled trip to Africa and various events at home fuelled intense speculation about his health, and in a July 2022 interview he acknowledged he needed to slow down.
At his weekly audience at the Vatican on Wednesday morning, just hours before his admission to hospital, the pope appeared in good spirits, smiling as he greeted the faithful from his "popemobile".
But he was pictured wincing as he was helped getting onto the vehicle -- a photo that made all the major front pages of Italy's newspapers on Thursday.
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Francis was admitted in July 2021 to the same Rome hospital for 10 days for an operation on his colon after suffering from a type of diverticulitis, an inflammation of pockets that develop in the lining of the intestine.
In an interview in January, Francis said the diverticulitis had returned.
Francis has repeatedly said, most recently in February, that he would consider stepping down if his health required, following the example of his predecessor Benedict XVI.
He has cautioned, however, that papal resignations should not be the norm, and said in an interview last month that the idea was currently not "on my agenda".
Asked by Italian Swiss television RSI in an interview broadcast on March 12 what condition would lead him to quit, Francis said: "A tiredness that doesn't let you see things clearly. A lack of clarity, of knowing how to evaluate situations".
Benedict, who died on December 31, shocked the world in 2013 by becoming the first pope since the Middle Ages to resign.
In 1957, at age 21, Francis had part of one of his lungs removed, but he has said he has made a full recovery.
Despite his health issues and advancing age, Francis — a Jesuit who seems most happy being among his flock — continues to travel internationally.
Earlier this year, he visited South Sudan and the Democratic Republic of Congo, drawing huge crowds.
Next month, he is due to visit Hungary and meet Prime Minister Viktor Orban.
In the past decade, Francis has sought to forge an image of a more open, compassionate Church, although has faced internal opposition, particularly from conservatives.
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