John Paul II, who celebrated his 84th birthday last May, has been in failing health for some time, although he has rarely had to cancel his official engagements.
The Pope has struggled on through his illnesses
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.
The appearance of the pontiff, with his bent frame and trembling hands, sometimes shocks pilgrims who attend audiences at the Vatican.
At times in the past 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.
He has visited more than 100 countries as Pope. Last August he celebrated an open-air Mass at the Roman Catholic shrine in Lourdes, south-western France, attended by 200,000 people.
POPE'S HEALTH SCARES
May 1981 - shot in stomach and hand during assassination attempt in St Peter's Square
July 92 - undergoes surgery to remove intestinal tumour
92 - symptoms of Parkinson's disease appear
Nov 93 - dislocates shoulder during fall from stairs
Apr 94 - suffers broken right thigh bone
Dec 95 - misses Christmas mass for the first time because of flu
March and Aug 96 - cancels public engagements because of fevers of "digestive nature"
Oct 96 - undergoes surgery to remove appendix
June 99 - cancels open-air mass in Poland after fall
March 02 - cancels public engagements because of flare-up of arthritis in right knee
Sept 03 - cancels engagements because of an intestinal ailment
Feb 05 - has week in hospital after falling ill with flu, readmitted after relapse two weeks later
Although he had to stop and ask an aide for some water, he insisted on finishing his sermon despite the 30C (86F) heat.
The Pope sees travel as an important part of his ministry.
"He decided at the start of his papacy that he would travel. He feels the call to be a pilgrim pope," Bishop Renato Boccardo, who has organised several papal trips, told the BBC last year.
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.
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 want John Paul II to call it a day.
In an open letter published before the Pope's visit to Switzerland last June, 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".
BBC correspondent Peter Gould says he has 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.