Carcinogens are agents, including certain chemicals, radiation, and infectious organisms, that can cause cancer. They may damage DNA, cause genetic mutations, or disrupt normal cellular processes, such as cell signaling and the regulation of cell growth. These changes can lead to uncontrolled cell proliferation and tumor development.

Although many causes of cancer are known, not every case has a single identifiable cause. Exposure to carcinogens can increase the risk of developing cancer, depending on factors such as the type of agent, the level and duration of exposure, and individual susceptibility.

Carcinogens include asbestos, benzene, certain nitrosamines, benzo[a]pyrene, formaldehyde, and some forms of chromium and other metals. Other chemicals, including acrylamide and malachite green, have different levels of evidence regarding their potential to cause cancer. Not every hazardous chemical is a proven human carcinogen; classifications depend on scientific evidence, and some substances are considered probably or possibly carcinogenic rather than definitively carcinogenic to humans.

In addition, some cancers are caused by infectious agents, including certain viruses, bacteria, and parasites. Infections were estimated to account for approximately 10% of new cancer cases worldwide in 2022. Important cancer-associated viruses include human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV), and Epstein–Barr virus (EBV), also known as human herpesvirus 4.

Tobacco remains the leading preventable cause of cancer worldwide, while cancer-causing infections are another major preventable risk factor. Viruses can contribute to cancer through several mechanisms, including producing proteins that interfere with normal cell regulation, causing chronic inflammation, suppressing immune function, or altering the host cell’s genetic material. Some cancer-associated viruses integrate their genetic material into the host genome, but this is not a universal mechanism.

In certain animal retroviruses, viral genes can directly activate cellular proto-oncogenes, which normally help regulate cell growth. This can lead to rapid cellular transformation, sometimes called acute transformation. Other viruses can contribute to cancer through more gradual processes, including persistent infection, chronic inflammation, or changes in gene regulation. In some cases, viral genetic material integrates into the host genome near a proto-oncogene or disrupts a tumor-suppressor gene, altering normal cell growth. However, integration is not required for all cancer-associated viruses, and the mechanisms and time required for cancer to develop vary by virus and type of cancer.

By Michael Chiu, MWS, CI – Updated and Revised in 2026