ROME — Pope Francis' respiratory tract 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 further change in his drug therapy. Scientists say polymicrobial diseases are caused by a mix of viruses, bacteria, fungi and parasites.