The breathing problems that Pope John Paul II is said to be suffering are not a routine complication of the flu but likely are worsened by the main health problem he suffers — Parkinson's disease (search), several doctors say.

"A normal person might get some tightness or constriction in the throat" but would not suffer spasms of the trachea, or windpipe, severe enough to necessitate hospitalization, said Dr. Jaimie Henderson, a Parkinson's expert at Stanford University School of Medicine (search).

The flu itself is cause enough for concern, especially considering the pontiff is 84 and frail. Flu and its main complication, pneumonia, kill nearly 40,000 elderly people in the United States each year. It is not known whether the pope had a flu shot or the pneumonia vaccine.

"His age and the fact he has Parkinson's disease puts him at higher risk of not recovering from this," said Dr. Gary Leo of the Regional Parkinson's Disease Center at Aurora-Sinai Medical Center (search) in Milwaukee.

"There are a lot of different processes that can make Parkinson's worse, and infections are one of them."

A Vatican spokesman said early Wednesday that the pope would spend several days in the hospital, where he had been taken as a precaution after he developed tracheitis (search) — throat spasms that made it difficult for him to breathe, a condition not unlike what many children go through with croup.

His heart function was said to be normal, but he has been increasingly frail from Parkinson's, an incurable neurological disease that gets progressively worse and weakens muscles that control breathing, swallowing and other movement.

"His breathing may not be as full because of the Parkinson's. The chest wall doesn't move as well. It may complicate his recovery," Leo said.

The disease puts him at high risk of potentially fatal complications, especially infections.

"There must be a suspicion that he's developed a complicating pneumonia," said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University (search). That would be an "ominous" sign for his health, Schaffner said.

A key sign in the coming days, doctors said, will be whether the pope requires a plastic breathing tube down his windpipe to help him breathe. That would require sedation and usually a machine to take over breathing for him until he is strong enough to breathe on his own again.