The 84-year-old Pontiff was rushed to Rome's Gemelli hospital late on Monday night fighting for breath after suffering what his spokesman, Joaquin Navarro-Valls, described as a "laryngo-spasm". He declined to give a date for the Pope's discharge, but said he would remain in the hospital "for another few days".
The Pope had fallen victim to the influenza epidemic that has swept Italy during the current spell of intensely cold weather, but the respiratory infection which complicated it and caused the breathing crisis is another unwelcome development, on top of the Parkinson's disease that has steadily robbed the Pope of mobility and eloquence. Medical experts said constriction of the chest muscles caused by Parkinson's can make it more difficult for sufferers to fight the infection.
The first sign of the Pope's new health problem came on Sunday when he greeted crowds in St Peter's Square before a window open to the freezing weather. His voice, which has become increasingly difficult to understand as Parkinson's takes its course, was croaky as he repeatedly tried to clear his throat.
John Paul missed his customary Wednesday audience for pilgrims in St Peter's Square yesterday for the first time since September 2003, when he was sick with an intestinal ailment. But the Vatican went to considerable lengths to downplay the significance of the crisis. Mr Navarro-Valls, a member of Opus Dei, insisted the Pontiff's condition had stabilised, and the condition of his heart and lungs were "within normal limits". He had at no point lost consciousness, he was not in intensive care nor had he required a tracheotomy to breathe.
John Paul took breakfast yesterday and participated in a mass at his bedside, but all impending engagements have been cancelled.
A Roman proverb goes, "the Pope is never sick until he's dead", and furious speculation has invariably accompanied bouts of papal illness. The present Pope, in keeping with his no-nonsense image, has been much more frank with his billion-strong global flock than any previous incumbent. He kept the world closely informed of his recuperation after the assassination attempt by the Turkish terrorist Mehmet Ali Agca, who shot him in 1981, and during the operation to remove a tumour described as benign but "as big as an orange" from his stomach in 1992.
Given the intense secrecy that still cloaks the innermost workings of the Vatican, and the Pope's evident frailty, anxiety and speculation will remain intense until he is visibly back to normal. Mr Navarro-Valls's insistence to reporters thronging the Gemelli hospital yesterday that "there is no reason for alarm today" will not calm the fears. Catholics around the world lit candles and said prayers for the Pontiff's recovery. The latest crisis is also bound to reignite speculation about who is likely to succeed the first Polish pope in history.
During his 27-year papacy, the second longest in history, John Paul has spearheaded a relentless fight by conservatives against the liberals and reformists who dominate the church in Britain and the United States, and most of the church's bishops and cardinals in office today are his own conservative-minded appointees. The latest turn of the conservative ratchet has seen Cardinal Josef Ratzinger, the Pope's personal theologian, nicknamed "Panzer Cardinal" for his hardline views and authoritarian instincts, being widely touted as a possible successor.
Vatican-watchers have generally regarded Ratzinger as too old, at 77, to succeed John Paul. But after the present incumbent's epic reign, odds have shortened on his being followed by another elderly prelate, to make the next papacy more likely to be relatively short.
Other favoured candidates include Cardinal Dionigi Tettemanzi, the Archbishop of Milan, 69, the Italian favourite, a natural mediator with few enemies, though Italians may no longer be able to dominate the Conclave, the meeting of cardinals after a pope's death that elects his successor.
The increasing strength of the Catholic Church outside Europe and North America has resulted in several non-Italians being discussed frequently as papabili ("pope-able"), including Cardinal Oscar Rodrigues Maradiaga, 60, a telegenic Honduran, and Nigeria's Cardinal Francis Arinze, 71, who would be the first black pope, if elected.
Austria's Cardinal Christoph Schonborn, 58, an intellectual heavyweight among the conservatives, could be a bridge between east and west but he is is considered too young by some and too similar to the present Slavic pope by others.
One possible outcome of John Paul's illness is still regarded as highly unlikely, the possibility that he might retire.