ROME – Pope Francis’ respiratory infection is presenting a “complex clinical picture” that will require further hospitalization, the Vatican said Monday, as concerns grew about the increasingly frail health of the 88-year-old pontiff.

Vatican spokesman Matteo Bruni said the results of tests conducted in recent days and Monday indicate the pope is suffering from a polymicrobial respiratory tract infection that has necessitated a second change in his drug therapy since being hospitalized Friday. Scientists say polymicrobial diseases are caused by a mix of viruses, bacteria, fungi and parasites. There was no timeframe given for his hospitalization, which at Day 4 has already sidelined Francis for longer than a 2023 hospitalization for pneumonia. Bruni said the complexity of his symptoms “will require an appropriate hospital stay.”

Francis had part of one lung removed after a pulmonary infection as a young man and is prone to bouts of bronchitis in winter. He was admitted to Rome’s Gemelli hospital in a “fair” condition on Friday after a weeklong bout of bronchitis worsened. Doctors confirmed a respiratory tract infection and prescribed “absolute rest” alongside unspecified drug therapies. Subsequent updates said his slight fever had gone away and that he was in “stationary” condition. Bronchitis, or an inflammation of the airwaves, can be relatively mild in a healthy person but can become much more severe in someone who is older or has existing lung problems, especially when they are unable to cough up and expel the accumulating mucus.