Kanga's treatment was successful, but the virus that struck her is causing increasing concern among some researchers who think it is causing a small-scale epidemic of throat cancer. That virus, scientists have proved only in the last two years, is human papillomavirus, or HPV - the same virus that's behind most cases of cervical cancer.

With 6,000 cases per year and an annual increase of up to 10 per cent in men younger than 60, some researchers say the HPV-linked throat cancers could overtake cervical cancer in the next decade. "It's almost a new disease, in a sense," said Dr Ezra Cohen, an oncologist at the University of Chicago Medical Centre. "It's now becoming a dominant sub-type of the disease that we see in our clinic." The HPV infections likely took root decades ago as the Baby Boomers were reaching adulthood, and only now are spurring a rise in throat cancer cases, mostly among men and women in their 50s.

No one understands the precise reason for the increase, though experts suspect it's linked to changes in sexual practices that emerged in the 1960s and '70s. For example, oral sex is a known risk factor for HPV-related throat cancers, and studies have shown that people who have come of age since the 1950s are more likely to have engaged in oral sex than those who were born earlier. "Those people were in their teens during the sexual revolution, so they may be leading the wave," said Dr Maura Gillison, a professor of oncology and epidemiology at the Johns Hopkins Kimmel Cancer Centre who has published numerous studies indicating that HPV-related throat cancer is a distinct type of disease.

The virus targets a specific portion of the upper throat called the oropharynx, which includes the tonsils and base of the tongue. Just a decade ago, doctors believed nearly all such cancers were linked with smoking or extremely heavy drinking. Last year, however, Gillison's team published a major study that found stark differences between the risky behaviours of throat cancer patients with HPV and those without. The HPV-positive cancer patients tended to have had higher numbers of sex partners than the others and were far more likely to have had multiple oral-sex partners. The virus-linked cancer appears somewhat less deadly than throat cancers that arise from smoking or drinking. A paper published this year found that 96 per cent of HPV-positive patients survived at least two years after diagnosis, compared with 62 per cent survival for HPV-negative cancers.

"They have a better prognosis, but these are still very aggressive cancers," said Dr Marshall Posner, medical director of head and neck oncology at the Dana-Farber/Harvard Cancer Centre in Boston. While doctors had hoped for an overall drop in throat cancer as the percentage of Americans who smoke declined, the rise of HPV-related throat cancers seems to be offsetting any such benefit.

Gillison's group tested hundreds of head and neck tumours that doctors began saving in the early 1970s, long before anyone knew such cancers might be linked to HPV. The work showed that the number of HPV-positive tumours increased by about 1 per cent a year on average, though the trend has quickened in the last decade, especially in men younger than 60. The virus thrives in the outer layer of skin and is transmitted mostly by skin-to-skin contact, researchers think. Sexual transmission tends to cause infection near the site of contact; intercourse is linked with cervical cancer, while oral sex can cause cancer of the upper throat. Recent increases in HPV-positive throat cancers could be an aftershock of changes in sexual behaviour that began decades ago. A landmark 1994 study of sexual behaviour showed that oral sex had become commonplace only in the generations born in the 1950s or later. Fewer than 60 per cent of people born in the 1930s were found to have had oral sex, compared with more than 80 per cent of people born since 1950.

"The older people simply did not do it as much," said Edward Laumann, a University of Chicago professor of sociology who led the National Health and Social Life Survey. "It's a very complicated social story that nobody's really worked out in detail," he said.

Other causes may have contributed to the spread of HPV cancers, including the increased movement of people around the country and the world in the last half-century, experts said. Scientists aren't even sure yet whether the virus might be spread by kissing, though data suggest oral sex is a major route of transmission. One implication of the cancer trend is that oral sex does not constitute "safe sex," Kanga said. "We can't be afraid to talk about this," Kanga said. "The message that oral sex carries risks is just not out there."

Conservative groups say the emergence of HPV-related throat cancer is an additional argument for abstaining from sex until marriage. But many researchers focus on expanded use of the HPV vaccine, which since 2006 has been recommended for girls ages 11 to 12. Just as the vaccine lessens the risk of cervical cancer for those girls, it may offer protection from HPV-positive throat cancer, though studies have not yet addressed that question.

Vaccine maker Merck & Co hopes to submit an application this year to the US Food and Drug Administration for use of its HPV vaccine in males. Although the company's studies will not show specifically whether the vaccine protects against throat cancer, they should reveal whether the shots prevent infection with HPV. "We expect the vaccine to work just as well in male and female populations," said Dr Richard Haupt, director of Merck's clinical program for the vaccine, Gardasil, which was developed using technology pioneered in Australia. MCT