The French and American co-discoverers of the Aids virus, Dr Luc Montagnier and Dr Robert Gallo, had intended to convince the Vatican to change its stance on condom use when they made an appointment to see Pope John Paul II in mid-June.

But when Dr Gallo was forced to drop out of the meeting for knee surgery, his opportunist French colleague took the time alone with the Pontiff to try a miracle of a different kind. Dr Montagnier told Le Monde yesterday he used the meeting to prescribe an extract from fermented Asian papayas for the 82-year-old Pope, who has the symptoms of Parkinson's disease.

The Vatican has never confirmed that the Pope suffers from a form of Parkinson's, but his symptomatic hand tremor and slurred speech have become increasingly evident.

Along with several biologists, Dr Montagnier believes that oxidative stress created when cells convert oxygen into energy contributes to neurodegenerative diseases such as Parkinson's. The fermented papaya extract contains antioxidants and stimulates the immune system, he said.

Dr Montagnier also prescribed a similar substance that is produced by a New York company, not named by Le Monde. Few dispute there has been an improvement in the Pope's health on recent trips to Canada and Poland, particularly evident in his power of speech. Reports yesterday from the papal residence of Castel Gandolfo said the Pontiff was "speaking strongly and clearly and joking with the crowd at his retreat in the hills south of Rome."

But whether this is due to Dr Montagnier's alternative medicine, the hand of God, or the benefits of a good holiday, remains unclear.

Dr Montagnier was the director of a Paris laboratory that identified the HIV virus in 1983. In the US, Dr Gallo helped to develop a blood test for the condition. The Aids researchers, once rivals, now work together to raise money to fund research into vaccines.