Two of the most common human viruses have just been added to the list of agents that health experts say can cause cancer. The National Institutes of Health added seven total new substances to its 14th Annual Report on Carcinogens this month — five viruses, a chemical, and a metallic element.

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The five viruses include human immunodeficiency virus type 1, human T-cell lymphotropic virus type 1, Epstein-Barr virus, Kaposi sarcoma-associated herpesvirus, and Merkel cell polyomavirus. Of the new agents, HIV and Epstein Barr (EBV), according to one cancer expert, are especially worth noting.

Cancer risk increases in people who are infected with these viruses if they also have a weakened immune system, said one cancer expert.

“There are approximately 1.2 million people in the U.S. who are infected with HIV, and EBV is one of the most common human viruses,” said Susan Olivo-Marston, Ph.D., an epidemiologist with The Ohio State University Comprehensive Cancer Center and College of Public Health.

“While infection with some of the other listed viruses is common in the United States, most infected people remain healthy and symptom-free. Cancer risk increases in people who are infected with these viruses if they also have a weakened immune system,” Olivo-Marson told LifeZette.

Approximately 12 percent of human cancers worldwide can be attributed to viruses. Health officials say that because 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.

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Cobalt and trichloroethylene are agents that the general population is likely not exposed to, or only exposed to at low levels.

“Cobalt has been added as ‘reasonably anticipated to be a carcinogen,’ which means that although there is some research in animals that suggest it can cause cancer, there isn’t enough research in humans to support this,” said Olivo-Marston. “High trichloroethylene exposure is also unlikely in the general population. Certain occupations — it is used as a metal degreasing agent — may have a higher risk of exposure, but it is believed that exposure in the general population is actually decreasing.”

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What still remains a real cancer threat to a healthy portion of our population is tobacco. It is still one of the leading risk factors for cancer, accounting for some 30 percent of all cancer deaths in the United States.

“There are many known carcinogens that people can control their exposure to, such as tobacco, sun exposure, and alcohol. A healthy lifestyle, including a healthy diet, a healthy weight, and physical activity are other important ways that people can reduce their cancer risk,” Olivo-Marston added.

The seven newly reviewed substances on the list of NIH’s official list of carcinogens brings the total to 248 listings.