Public appearances on a wheeled throne
The fragile health of John Paul II made many people wonder if he would complete the 25th year of his papacy.
Not for the first time, his resilience has confounded the doubters. But the anniversary celebrations will not diminish speculation about his ability to carry on.
The Pope suffers from Parkinson's Disease and severe arthritis, and now makes all his public appearances on a wheeled throne.
Under Church law, he can resign. But over the centuries, most popes have died in office, rather than stand down.
Church faces dilemma
Father Thomas Reese, author of the book Inside the Vatican, believes it is a tradition that could - in the modern world - face the Church with a dilemma.
"Suppose the Pope goes into a coma, or suppose a Pope becomes mentally unbalanced, or suffers from Alzheimer's?" he says.
"If it gets to that kind of a situation where the Pope is very sick and can't function, but he's not even well enough to resign, then we don't know what to do. In the Catholic Church there is no procedure for dealing with a pope who becomes incapacitated.
"This could cause a major constitutional crisis in the Church, because medical technology today can keep someone alive way beyond the point where they are capable of functioning."
Father Reese believes the Pope would be prepared to stand down if his health prevented him from doing the job. But some experts think a papal resignation could create other problems for the Church.
Marco Politi, one of Italy's leading commentators on the papacy, says the Vatican has discussed the option.
"A lot of people were saying that from a humanitarian point of view, to have the Pope continue doing his job was to impose a torture on him - it was unfair," he says.
"The Pope submitted the question to a secret group of advisers, who studied the issue. They said the Catholic faithful were not ready to accept the contemporary presence of two popes - one retired Pope, and one acting Pope."
The problem of having two living popes is also highlighted by John Wilkins, the editor of The Tablet.
"At present, popes leave office because they die," he says.
"At the moment they die, everything is open. You are totally free to choose someone who may have very different ideas and a very different way of carrying out things.
"If you had a resigned Pope - regarded by many as a living saint - somewhere in a monastery in the mountains, it would be almost impossible to choose someone who might have a different line to follow."
In time, the Church may come to accept that popes should resign at a certain age. But for now, what happens if the present Pope becomes too ill to perform his duties?
John Wilkins believes John Paul II may have anticipated such an eventuality.
"There is a story that the Pope has written a letter, which I presume he will have entrusted to his Polish secretary, which says: 'If at a certain point I become unable to exercise my faculties, my papacy must be considered at an end.'
"I consider it quite possible that there is such a letter, but all this points to the need for a rather better way as people live older, we do need to look at the procedures for a pope's reign."
But in the absence of any such machinery, he says, the running of the Church would be in the hands of the pope's aides.
"You would have to go on managing the Church by a number two or a number three, or a group, until that pope dies," he says.
"There is no provision at the moment for a pope who becomes incapable and disabled, and who doesn't know where he is, there is nothing you can do."
Father Thomas Reese agrees that in those circumstances, the Church would have only one option.
"I think it would pray, and hold its breath, and probably just wait for him to die," he says.
"There are some canon lawyers who have come up with theories on the ways the Church could handle this, but there are differences of opinion.
"What is really needed is for the Pope himself to promulgate legislation which would say: 'This is the way the Church should deal with this if I become incapacitated and incapable of governing.'
"Nobody in Rome really wants to talk about that kind of thing, let alone suggest it to the Pope, but that is the kind of thing that is needed.
"He might have something like this in writing that is being kept secret and would only be released if he became so incapacitated that it would go into effect.
"That is not a very good way to do law, but it may be the way he has handled it."