One young researcher disagreed. Harald zur Hausen, who studied viruses and cancer at the University of Pennsylvania, staked his career on another possibility: the human papilloma virus (HPV).

After persisting for more than a decade, he proved his case. "That link was the finding that allowed development of the vaccine" Gardasil, which prevents most cervical cancers, said Peter Kim, president of Merck Research Laboratories.

Yesterday, zur Hausen, now an emeritus professor at the German Cancer Research Center in Heidelberg, was one of three scientists who won the Nobel Prize in medicine. With the HPV vaccine now being distributed to millions of females, his finding promises to make inroads against cervical cancer, which kills 250,000 women every year, mostly in the developing world.

"I'm not prepared for this," zur Hausen, 72, told the Associated Press yesterday. "We're drinking a little glass of bubbly right now."

It was the second year in a row that a scientist with Philadelphia ties won a Nobel Prize in medicine. Mario Capecchi, an Italian street urchin after World War II who emigrated to Bucks County and graduated from the George School, shared the 2007 prize for his work in targeting genes.

Peter Howley, who chairs Harvard University's pathology department, said it was not just zur Hausen's perseverance and insight but his collaborative spirit that helped make the vaccine possible.

"He and his colleagues were very generous in making their [ideas] available to the community," Howley said.

Zur Hausen came to Penn from Dusseldorf, Germany, in 1965, recruited by Werner and Gertrude Henle to research the link between another virus, Epstein-Barr Virus (EBV), and a cancer known as Burkitt's lymphoma.

In an interview with The Inquirer in 2006, zur Hausen recalled that he could barely speak English at the time.

"Gertrude was talking enormously and flooding me with information about a virus I had never heard of before," he said. "I tried to hide my ignorance. Anyway, they decided to accept me."

While doing groundbreaking work showing how EBV could invade human DNA, zur Hausen started thinking about cervical cancer.

For decades, scientists had noted evidence that cervical cancer was connected to a sexually transmitted pathogen. Nuns, for instance, rarely if ever got the disease.

Genital herpes was a focus at the time, as the sexual revolution was aiding its spread.

Zur Hausen, however, had been looking at other work connecting abnormal cervical cells in pap smears with the genital-warts virus, HPV. He moved back to Germany in 1969, but continued with his quest.

He eventually isolated a genital-warts virus - HPV6 - but realized it was not associated with cancer.

Zur Hausen kept looking and eventually found a previously unknown HPV strain, HPV16, from cervical cancers.

In the early 1980s, he and a junior colleague were able to use cloning to make many copies of HPV16. They also found it in many samples of cervical cancer. They soon discovered a second new strain, HPV18, also strongly linked to cancer.

It took many more years for him and other scientists to complete epidemiological studies showing that these two viruses were responsible for about 70 percent of all cervical cancers.

It wasn't an easy road, zur Hausen recalled in 2006. Other scientists "just simply didn't believe me," he said. "The memory I have is of enormous disbelief."

But by the mid-1980s, he had convinced the skeptics.

The Nobel "is very well-deserved," said Carlo Croce, head of human cancer genetics at Ohio State University. He said the vaccine could lead to the disappearance of cervical cancer.

It doesn't stop there. HPV also is associated with penile cancer, vaginal cancer, head and neck cancer, and several others, said Erle Robertson, a microbiology professor at Penn.

Still, he said, the vaccine does not confer total protection. About 30 percent of cervical cancers are caused by other, rarer strains of HPV.

"There's still more work to be done," Robertson said.

Since the approval of the HPV vaccine in June 2006, Merck has delivered more than 30 million doses of its version, called Gardasil, according to a spokesman.

Merck's Peter Kim said it took so many years to develop the vaccine because scientists needed to do long clinical trials to prove it prevented the disease.

The company is now testing the vaccine in men, he said. Vaccinating males might help cut the prevalence of the viruses, which are so ubiquitous that most women have picked up a sexually transmitted HPV at one time or another.

For reasons scientists do not understand, these HPVs usually clear up on their own. But in a few cases, they persist long enough to cause cancer many years after the initial infection.

In the United States, women with HPV are still unlikely to die from cervical cancer if they get pap smears every year. Doctors recommend they continue to do so.

But in other countries, screening and treatment aren't available, which is why 250,000 women still die a slow and painful death from the disease.

"I have a son and a daughter," Penn's Robertson said. "When the time comes, I want them both to get the vaccine."

Audio of a phone interview with Harald zur Hausen, the story of his quest to find a cancer vaccine, and related links: http://go.philly.com/health

at 215-854-4977 or fflam@phillynews.com.