- Human immunodeficiency virus type 1 (HIV-1)
- Human T-cell lymphotropic virus type 1 (HTLV-1)
- Epstein-Barr virus (EBV)
- Kaposi sarcoma-associated herpesvirus (KSHV)
- Merkel cell polyomavirus (MCV)
These viruses have been linked to more than 20 kinds of cancer, according to the report, including non-melanoma skin cancer, eye cancer, lung cancer, stomach cancer and multiple types of lymphoma.
"Given that approximately 12% of human cancers worldwide are attributed to viruses, and there are no vaccines currently available for these five viruses, prevention strategies to reduce the infections that can lead to cancer are even more critical," said Linda Birnbaum, director of the National Institute of Environmental Health Sciences and the National Toxicology Program, in a statement.
"HIV/AIDS, for example ... attacks the body`s immune system, which plays an important role in surveillance against cancers," Birnbaum told CNN. "If you have a weakened immune system, you`re more likely to be at risk for certain types of cancers."
Additionally, trichloroethylene, or TCE -- "an industrial solvent primarily used to make hydrofluorocarbon chemicals" -- is now classified as carcinogenic to humans.
"There are many ways people can be exposed to TCE," according to the report. "It can be released into the air, water and soil at places where it is produced or used. It breaks down slowly and can move readily through the soil to make its way into underground drinking water sources. Because of its widespread use as a metal degreasing agent to maintain military equipment, it has been found in the groundwater at many military and Superfund sites."
The metallic element cobalt and cobalt compounds that release cobalt ions in vivo -- or, inside the body -- are now classified as "reasonably anticipated to be a human carcinogen."
"Cobalt is a naturally occurring element used to make metal alloys and other metal compounds, such as military equipment and rechargeable batteries," according to the report. "The highest exposure occurs in the workplace and from failed surgical implants."
Cobalt`s classification in this report "does not include vitamin B12, because cobalt in this essential nutrient is bound to protein and does not release cobalt ions."
Even if you are exposed to something known to be carcinogenic -- whether it be a virus, a chemical or radiation -- it does not necessarily mean you are going to develop cancer.
"Most people now know that smoking causes lung cancer, but only 11% of smokers get lung cancer," Birnbaum said. "Some people are more susceptible because of genetic, lifestyle, nutrition or age when the exposure occurs."
Still, cigarette smoking is the No. 1 risk factor for lung cancer. "In the United States, cigarette smoking is linked to about 80% to 90% of lung cancers," according to the Centers for Disease Control and Prevention. "People who smoke cigarettes are 15 to 30 times more likely to get lung cancer or die from lung cancer than people who do not smoke."
Bottom line: Researchers recommend that people talk to their health care providers about minimizing their behavioral risks. When it comes to the new viral additions to this list, that largely means practicing safe sex and abstaining from sharing needles.
"Grandma said, `an ounce of prevention is worth a pound of cure,` " Birnbaum said. "I`m a big believer in that."
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