Doctors say the latest health crisis will be a challenge for Pope John Paul II (search) to overcome, given the number of ailments weakening his frail body.

The 84-year-old pontiff, who began receiving nutrition through a feeding tube on Wednesday, developed a high fever Thursday because of a urinary tract infection (search). The pope was immediately put on antibiotics at the Vatican and was said to be in stable condition.

"It's not a very promising situation," said Dr. Benjamin Ansell, an internist at UCLA School of Medicine (search). "When you see recurrent infections as the pope has had, each round of antibiotics may lead to resistance."

Ansell said a healthy person may recover from a high fever with no problem, but it could be devastating for those with neurological disorders such as Parkinson's disease, which the pope has suffered for at least a decade. Some Parkinson's patients who develop a fever may turn catatonic, Ansell said.

John Paul's 26-year papacy has been marked by health ailments that have left him frail and forced him to cancel public appearances. The latest illness comes just a month after breathing problems forced the pope to undergo surgery to insert a tube in his throat to help with his breathing.

Besides Parkinson's, the pope suffers from knee and hip problems that make it impossible to stand. In 1992, he had surgery to remove a benign tumor in his colon. Four years later, he had his inflamed appendix removed.

"His body has come to a standstill," said Dr. Zab Mosenifar, who treats elderly patients at the intensive care unit at Cedars-Sinai Medical Center in Los Angeles. "Usually, these people go in a downhill course."

Mosenifar noted that the body's organs are interdependent on one another and if one system fails, it could cause a "cascade effect" of other systems shutting down.

"It becomes a vicious cycle," Mosenifar said.

The pope was being treated at the Vatican. At the Gemelli Polyclinic, the hospital where the pope has been treated before, an emergency room chief said there were no immediate plans to admit John Paul. Medical experts say despite not being admitted to the hospital, John Paul was most likely to get the best medical treatment where he currently is.