While the Vatican (search) expresses optimism Pope John Paul II (search) will rebound from his latest health crisis, outside medical experts give a grimmer long-term view, saying the pontiff's Parkinson's disease puts him on the path of a steady — and perhaps rapid — decline.

The 84-year-old pope was rushed to a hospital Tuesday with breathing problems. Vatican officials say he has the flu and is being treated for a swollen windpipe and spasms of the voice box. They announced Friday the pope's condition had stabilized and his breathing had improved.

The reported problems are common among patients with advanced Parkinson's disease (search). Often they are caused by a chest infection such as pneumonia, triggered by swallowing mishaps where food or saliva enter the lungs instead of the stomach.

The Vatican has not revealed whether the pope's medical crisis is linked with such a scenario, but experts say such incidents are increasingly likely to happen to him.

"It's very hard to be able to predict for any given individual where they are going to ultimately be unless you know their case very well, but ... it does seem very likely he will continue to have these kinds of problems," said Parkinson's expert Dr. Michael Kaplitt, director of movement disorder surgery at New York Presbyterian Hospital in New York.

He said judging from his symptoms, the pope could succumb to Parkinson's or to a disorder complicated by Parkinson's at any time. "It could be a week, it could be a year and a half. There's really no way to tell," he said.

Parkinson's occurs when cells in the part of the brain that controls movement are lost. These cells produce dopamine, a chemical messenger that enables people to perform smooth coordinated movements.

The disease gets worse over time, but the speed of decline varies from person to person. About 10 percent of patients will seem no different after 10 years, but 25 percent end up in a wheelchair or bedridden.

It usually starts with shaking of the hands or feet, muscle stiffness and slowness of movement. Sufferers often find it hard to get up out of a chair or roll over in bed and take small shuffling steps. Half of sufferers have speech problems.

"Usually it's a loss of volume, intonation and the ability to control the rhythm of your voice," said Sheila Scott, a speech therapist at the Parkinson's Disease Society, based in London. "The words can all run together and that can make it very difficult to distinguish."

This happens because of rigidity in the muscles of the diaphragm, the voice box, face, lips and tongue. The problem gets worse as the disease progresses and sometimes patients end up incoherent much of the time. The pope has been slurring his speech for several years.

Problems with the same muscles are also involved in swallowing difficulties, which make it easy for food or saliva to get into the lungs. That can cause a life-threatening pneumonia and is one of the most common causes of death among Parkinson's patients.

Sometimes a feeding tube is inserted directly into the stomach to help patients who have difficulty eating and to help prevent food entering the lungs.

The muscle problems also make it difficult for patients to head off infections by mustering a powerful enough cough to bring up mucus from the lungs, especially if their posture is stooped, as the pope's is.

Dementia is also common, and gets worse as the disease progresses.

About 4 million people worldwide are estimated to have Parkinson's. Well known sufferers include the boxer Muhammad Ali (search) and actor Michael J. Fox (search).

Although John Paul has had symptoms of Parkinson's for at least a decade, the Vatican has never officially attributed the pontiff's slurred speech and trembling hands to the disease. It was only in May 2003 that a top Vatican cardinal, Giovanni Battista Re, said in a newspaper interview that the pope suffered from the disorder.

Drugs are the main treatment. Some patients can be helped by surgery placing electrodes in the brain to smooth out the signals to the muscles. However, the pope's Parkinson's is too advanced to benefit from such surgery, said Kaplitt.

"We're only guessing, and anything's possible, but his prognosis is obviously not great," said Dr. Eric Braverman, Director of PATH Medical clinic in New York. "Pre-illness function predicts post-illness recovery and his functioning wasn't great to start with."