US researchers have found the risk of infection with the human papilloma virus (HPV) is increased ten-fold for each new sexual relationship a woman has per month.
Their findings suggest that more than half of sexually active young women in the US will become infected with HPV over a three-year period. The infection figures are thought to be similar in Europe.
However, the same study has contradicted previous research which suggests that infection with the virus is certain to trigger cellular changes that can lead to cancer.
Three in ten
The researchers found that, over a five-year period, only approximately 30% of women infected with HVP showed signs of changes to the cells of the cervix.
These changes, called LSIL, or low-grade intra-epithelial lesions, are benign in themselves, but can be the first step along the way to the development of cervical cancer.
It appears that women are more likely to develop LSIL in the first year after HPV infection. The risk tails off after four years.
Infection with other sexually transmitted diseases did not, as indicated by previous research, increase the risk of LSIL.
However, daily cigarette smoking did appear to increase the risk.
Lead researcher Professor Anna-Barbara Moscicki said: "This report shows that HPV is necessary but not sufficient by itself to cause these lesions.
"Other behavioral and biological factors must be involved, most likely persistence of the viral infection, and the woman's own immune reaction."
Long-term study
Professor Moscicki's work has overturned a number of assumptions about HPV.
Most strikingly, the researchers have shown that for most young women, HPV is not necessarily a life-long infection. Most appear to clear the virus from their bodies.
The research has also shown:
A spokesperson said: "HPV is a common sexually transmitted infection which in some women will lead to cancerous cervical lesions.
"Prevention is the key to tackling such infections and this study reinforces the importance of safe sex and use of condoms."
A spokeswoman for the Cancer Research Campaign said the study, which matched results found in similar British research, reinforced the need for safe sex precautions to stop the spread of the virus.
"What we need to find are the other factors which, when added to HPV infection, produce these changes."
Dr Peter Sasieni, a cervical cancer expert at the Imperial Cancer Research Fund, said: "There is great optimism that a vaccine against this virus will one day eliminate cervical cancer and do away with the need for regular cervical screening."
The research is published in the Journal of the American Medical Association.