While Pope Benedict XVI might be the first Holy Father to voluntarily resign because of old age and deteriorating health, the papacy has a long history of health ailments -- from depression to gout to cancer.
According to church law, as long as a pope is able to conduct Mass, he can continue to do so – even if he is suffering the pain of failing kidneys and is otherwise bedridden, like Pope Alexander VII was in 1667.
Pope Alexander VII's surgeon and confessor tried to persuade him not to go before the crowd on Easter Sunday of 1667, but he did it anyway. The pope died three days later, according to "The Deaths of the Popes," by Wendy J. Reardon.
More than a century later, Pope Clement XIV complained of pain and wasting away before he became known as the pope who drooled and had eyes that "darted in their bulging sockets" as he fearfully clung to walls for fear of a Jesuit assassination attempt, according to Reardon. He died after correctly predicting his own death in 1774.
Then, in 1958, Pope Pius XII died after enduring hiccoughs and strokes for five years. The hiccoughs became so intense, that they tore the lining of his stomach, Reardon wrote. He died of complications from pneumonia at 82 years old.
Still, the popes stayed the course until their deaths.
"Death has never been an issue that has worried popes," said papal historian Anura Guruge, a former IBM employee who's spent the last six years researching the papacy and entering related data into more than 70 spreadsheets for some first-of-its-kind analysis. He's written two books on the subject.
"Popes talk about no purgatory for popes," he said. "They immediately go to God's house and they're immediately in the presence of the father. Many of the popes on their death beds expressed enormous amounts of joy."
With only seven voluntary resignations out of more than 265 legitimate popes, ailing and death have become part of the process, Guruge said. If God is ready for a new pope, he will simply call the current one home to heaven.
"This pope's resigning is essentially overriding God's will," he said, adding that Pope Benedict XVI's resignation was a surprise to him. "We had suspected that he had more health issues than had been made public. ... A pope resigning is really not the right thing to do."
Pope Benedict XVI, who was one of the oldest popes when he was elected in 2005 at age 78, had a stroke in 1991 that reportedly temporarily affected his vision. He fell in 1992 and 2009, and also had either arthritis or arthrosis, a similarly painful and debilitating joint condition. He was spotted using a cane during trips to Mexico and Cuba last year. He also used a mobile platform to get around St. Peter's.
The pope's brother, George Ratzinger, told the U.K.'s Telegraph that the pope's doctor advised him against taking any more transatlantic trips.
"His age is weighing on him," Ratzinger, 89, told the Telegraph, adding that the pope had been considering stepping down. "At this age, my brother wants more rest."
Father Virgilio Elizondo, a professor at the University of Notre Dame in Indiana, said he thinks Pope Benedict XVI made a very difficult but wise decision by resigning. He added that the papacy has a history of unpredictability, and the surprise resignation fits right in.