And seven of those were just added.

Five of the new substances added are actually viruses that either cause cancer directly or make it easier for other cancer-causing viruses to do their deadly work. They include: 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). All five of these viruses are classified as "Known to be a human carcinogen," as opposed to the agency's other category of "Reasonably anticipated to be a human carcinogen."

Viruses might not commonly be thought about as cancer-causing substances, but according to the National Institutes of Health (NIH), they are a real threat.