(CNN)-- One in six cancer cases worldwide are caused by infections, many of which are preventable or treatable, according to a study published Tuesday in the journal The Lancet Oncology.
A total of 2 million new cancer cases in 2008 were linked to infections, the study said. Of those, only 7.4% were reported in more developed countries, and 22.9% in less developed countries.
The research blames many of these cancer cases on Human papillomavirus, hepatitis B, hepatitis C and the stomach bacterium Helicobacter pylori.
HPV is preventable through vaccines approved by the U.S. Food and Drug Administration for girls and boys age 9 through 26. Merck's Gardasil is available for both sexes; GlaxoSmithKline's Cervarix is currently only approved for girls. While this common sexually transmitted disease doesn't cause cancer in most people, it can lead to cervical and some head and neck cancers.
"The more people you vaccinate, male and female, the more likely you are to get a population that doesn’t have the disease," said Dr. Otis Brawley, chief medical officer of the American Cancer Society and conditions expert for CNN Health, who was not involved in the study.
Hepatitis B is preventable by vaccine; hepatitis C is not, but treatments are available. Both forms used to be problematic in the United States mainly among IV drug users; today, sexual transmission is a bigger problem, Brawley said. These diseases lead to inflammation, scarring and regeneration of the liver, which can all cause cancer.
Helicobacter pylori can be treated with an antibiotic cocktail. Treating the bacterial infection has not been proven to prevent gastric cancer, but that is the hope among clinicians, Brawley said.
HIV is another disease that leads to forms of cancer, such as lymphoma and leukemia. That's because suppression of the immune system increases cancer risk - and that includes transplant patients who have to take immunosuppressant drugs so their bodies don't reject the donated organs.
There were big differences in the study between types of cancers that infected women and men. About half of infection-linked cancers seen in women were blamed on cervix uteri cancers. Among men in the study, more than 80% of cancers tied to infection were liver and gastric cancer.
But the total number of infection-related cancer cases was similar in men and women, a pattern that was consistent across age groups, except among people younger than 40. Women under 40 had more infection-linked cancers than men, mostly because of cervical cancer, the study said.
In terms of deaths, the study authors estimated that 1.5 million of the 7.5 million cancer deaths that occurred worldwide in 2008 - or about one in five - were related to infectious diseases.
How do researchers know if a cancer is caused by an infectious disease? Viruses such as HPV and Hepatitis B and C actually invade a person's DNA and leave their signature in the genetic sequence. Helicobacter pylori does not, but the bacterium can be found in gastric tumors.
There are some limitations to the estimates in the study, however. Since data by region and country wasn't always available, researchers can only estimate the number of infection-linked cancers in those areas. The authors could have overestimated infections in certain regions because of their reliance on large studies and those done in high-risk countries. It's also true that multiple infections can cause a single case of cancer, says Dr. Goodarz Danaei of the Harvard School of Public Health in an accompanying commentary.