By Peter Gould
BBC News Online correspondent
The visit by US President George W Bush to the Vatican has once again focused attention on the frail health of John Paul II.
The Pope, who celebrated his 84th birthday last month, arrived for the audience on a wheeled throne.
The Pope looked frail as he pronounced his speech to Bush
He has suffered from Parkinson's disease for several years, and painful arthritis in his legs means he now finds it difficult to stand unaided.
As a courtesy to President Bush, the Pope spoke in English, but his slurred speech made it difficult for those listening to make out some of the words.
The appearance of the pontiff, with his bent frame and trembling hands, sometimes shocks pilgrims who attend audiences at the Vatican.
At times over the past two or three years he has had difficulty completing speeches, and tires very quickly during public appearances.
Vatican officials finally persuaded a reluctant Pope that he would have to accept the limitations imposed by his illness, and spend more time resting and cutting back on his trademark foreign tours.
His visit to Slovakia, in September 2003, was his 102nd journey overseas. The obvious strain on the pontiff led observers to wonder if it might be his last.
But the Pope's visit to Switzerland this weekend demonstrates that he has not yet abandoned his travels.
However, this trip involves a fairly short journey, and limited public appearances.
Although the Pope's health is a cause of concern for the Catholic faithful, he has, remarkably, appeared to find new energy in recent months.
The Vatican does not comment on his medical treatment, but there has been speculation that the improvement in the Pope's condition in recent months is the result of a new regime.
One factor must be the extra time now allowed for rest and recuperation.
After the huge public celebrations last October, to mark his silver jubilee, the Pope had a relatively quiet few months.
The benefit was evident at Easter, when he appeared less tired than in previous years as he took part in the traditional ceremonies at the Vatican.
While some Catholics have wondered whether it might be time for the Pope to retire to a monastery in the Polish mountains, he has made it clear he has no intention of standing down.
The view of John Paul II is that he was given the job by God, and it will be God who decides when the papacy ends.
In this age of television, some wonder about the wisdom of projecting an image of an ailing Pope, struggling to complete public appearances.
A softly-spoken Pope had tough words for Bush
But Vatican observers point to the enthusiasm with which his appearances are greeted, particularly by the young, and the obvious compassion for a man who refuses to be stopped by his disability.
Yet some Catholics in Switzerland want John Paul II to call it a day.
In an open letter published before this weekend's papal visit, theologian Xaver Pfister said the Pope should respect the normal retirement age for bishops, set by the Vatican at 75.
"The media only talk about the Pope's health and no longer about what he says, which creates a credibility problem for himself and the papacy," he said.
But the Bishop of Basel, Kurt Koch, said the timing of the letter was "tasteless and perfidious" and described the suggestion that the Pope should retire as "absurd".
I have seen the Pope many times since his election in 1978, and even in sickness he retains an extraordinary power to draw a crowd.
On his last visit to his native Poland, two million people turned up for an open-air mass in Krakow.
When he makes his regular Sunday appearance at his window above St Peter's Square, the pilgrims below weep and applaud wildly.
Highlights of the Pope's trip to Switzerland this weekend include a meeting with Catholic youth groups at an ice rink, and an open-air mass.
He will stay in a retreat run by nuns, with special facilities for the disabled.
Further overseas trips are not ruled out. It is thought that in August he could visit the French shrine of Lourdes.
Although the Pope's physical condition causes him pain, his aides say he remains mentally sharp and his enthusiasm is undiminished.
"He is aware that travel is still part of his ministry," says Bishop Renato Boccardo, who organises papal trips.
"He decided at the start of his papacy that he would travel. He feels the call to be a pilgrim pope."