Like many other elderly people with chronic illnesses, Pope John Paul II suffered a cascade of organ failures triggered by a simple bacterial infection.
Crippled by advanced Parkinson's disease and still weakened from a recent bout of the flu, the 84-year-old pope was especially vulnerable to infection -- and later unable to fight its progression.
Each time doctors treated one problem, a new complication arose. A urinary tract infection became a blood infection, which attacked the kidneys and strained the heart and lungs.
"Once this multisystem organ failure starts happening, you really can't come back," said Dr. Benjamin Ansell, an internist at the University of California, Los Angeles, Medical Center.
Several medical experts who followed news reports of the pope's decline said it appeared that he had opted for a relatively comfortable death rather than aggressive treatment that might have briefly extended his life.
While a feeding tube was inserted through his nose, he remained at his Vatican apartment, where it would be difficult to provide dialysis to take over for his failing kidneys or place him on a ventilator to aid his breathing.
"At some point he -- or somebody who knew his directive -- said, 'We are going to go this far and no further,'" said Dr. Leslie Saxon, a cardiologist at the University of Southern California's Keck School of Medicine.
During the last decade, the pope had slowly deteriorated from Parkinson's, an incurable illness, which destroys brain cells that produce the chemical dopamine. The disease's effects include memory and cognitive impairment, trembling hands, gradual loss of muscle control and a stooped posture that compresses the lungs, making breathing more difficult.
Parkinson's patients "can't expand their chests as well" as the healthy can, said Dr. Jeff Bronstein, a neurologist at UCLA Medical Center. "It interferes with their ability to clear their lungs. It is harder to get over the infections."
In February, after the pope was diagnosed with the flu, doctors performed a tracheotomy, inserting a tube through a hole in his throat to help him breathe and make it easier to suction out saliva and debris that can enter the lungs and cause pneumonia.
The tube carries its own risk, because it can make swallowing harder and decrease appetite. With less nutrition, a patient is more vulnerable to infection.
The pope's rapid decline this week began with a urinary tract infection -- an ailment common among older men, who often have difficulty completely emptying their bladders.
Such infections usually are treatable with antibiotics. But with poor nourishment, weak lungs and muscles incapacitated by Parkinson's, the pope could not stop its advance.