Roughly one in six cancers is caused by an infection, according to a global study highlighting the power of vaccines in cancer prevention.

French researchers pooled data on 27 cancers from 184 countries to calculate the fraction of cases attributable to viral, bacterial and parasitic infections.

"Around 2 million cancer cases each year are caused by infectious agents," the researchers wrote in their report, published today in The Lancet Oncology. "Application of existing public health methods for infection prevention, such as vaccination, safer injection practice, or antimicrobial treatments, could have a substantial effect on the future burden of cancer worldwide."

Human papillomavirus (HPV), hepatitis B and C, and the ulcer inducing Helicobacter pylori caused 1.9 million cancers worldwide in 2008, according to the study. HPV and hepatitis B infections are largely preventable through vaccination, and H. pylori can be treated with antibiotics.

"Most of the infection-attributable cases occurred in less developed countries and were due to preventable or treatable infections," Goodarz Danaei, assistant professor of global health at the Harvard School of Public Health in Boston, wrote in an editorial accompanying the study. "Since effective and relatively low-cost vaccines for HPV and [hepatitis B] are available, increasing vaccine coverage should be a priority for health systems in high-burden countries."

HPV is a sexually transmitted infection that causes cervical cancer as well as cancers of the throat, vagina, vulva, anus and penis, according to the U.S. Centers for Disease Control and Prevention. The CDC recommends the HPV vaccine for teenage girls and boys as well as some men and women up to age 26, but few end up getting all the necessary doses.

"Our vaccination program is gaining momentum but very slowly, and one reason is it's hard to get teenagers in for all three doses," said Dr. William Schaffner, chair of preventive medicine at Vanderbilt University Medical Center in Nashville, Tenn. "The other reason is that because HPV is sexually transmitted, it's evoked a whole bunch of hullaballoo over whether the vaccine promotes promiscuity. Of course, there's no evidence to support that at all."

The hepatitis B vaccine is also given in three doses, but in the first 18 months of life.

"We vaccinate all children against hepatitis B, so their risk of liver cancer down the road will be very much reduced," said Schaffner. "If we look back 20 years from now, we will see the occurrence of liver cancer dropping precipitously."

The realization that infections like HPV can trigger cancer is relatively new, earning virologists Harald zur Hausen, Françoise Barré-Sinoussi and Luc Montagnier the Nobel Prize in 2008.

"Every time we make and advance like that, the opportunity exists to make a vaccine that could prevent those kinds of infections and thus prevent another proportion of cancers that occur in our population," said Schaffner.

But, he added, choosing not to smoke, eating a healthy diet and keeping physically active also reduce the risk of cancer.

"We have to remember that in our country behavioral risk factors still loom large," he said. "There are a number of strategies we can all employ to reduce our risk of cancer even more."