All of these conditions make each other worse. But the pontiff's current woes don't necessarily mean he faces a downward spiral of steadily worsening health, although they do make it very tough for him to recover.
A Vatican spokesman said the pope had been hospitalized because he had the flu and developed throat spasms that made it difficult for him to breathe — a condition not unlike what many children go through with croup.
"I would expect it to resolve over time," said Dr. Jaimie Henderson, a Parkinson's expert at Stanford University School of Medicine. But he added, "Parkinson's is a progressive disease. People usually die from other complications," and infections are at the top of the list.
"His underlying medical conditions really do put him at the highest risk for developing complications of all sorts. We know that Parkinson's disease leads to complications, including difficulty swallowing or eating," said CBS News Early Show medical correspondent Dr. Emily Senay. " …muscle rigidity, the inability to move muscles in a way that you want to and obviously, balance problems, tremors. These are all side effects of the muscle problem that comes from Parkinson's disease, which is a brain disease that affects the entire body and your ability to move your muscles in the way that you want to."
"There must be a suspicion that he's developed a complicating pneumonia," said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University. That would be an "ominous" sign for his health, Schaffner said.
The Vatican statement doesn't say whether the pontiff's illness was confirmed as flu, or whether he had had a flu shot or pneumonia vaccine, which prevents the most fatal complication of flu.
The Vatican said in an earlier statement that the pope suffered from "an acute laryngeal tracheitis and larynx spasm crisis."
Tracheitis, an inflammation of the trachea, requires hospitalization and usually a breathing tube to keep the airway clear. The throat spasms are likely a complication from the respiratory illness he has had.
It's possible his Parkinson's disease has made his condition more serious and his breathing more labored.
"A normal person might get some tightness or constriction in the throat," but would not suffer spasms, Henderson said.
"His breathing may not be as full because of the Parkinson's. The chest wall doesn't move as well," said Dr. Gary Leo of the Regional Parkinson's Center at Aurora-Sinai Medical Center in Milwaukee, Wis. "It may complicate his recovery."