Some carcinogens are living organisms. Certain viruses, bacteria, and parasites account for about 15 percent of all cancer deaths in the United States. Cancer-causing viruses include the human papilloma virus (HPV), a sexually transmitted virus responsible for 70 to 80 percent of all cases of cancer of the cervix.
The hepatitis B virus (HBV), recognizable under magnification by the round, infectious “Dane particles” accompanied by tube-shaped, empty viral envelopes, causes nearly 80 percent of liver cancers worldwide.
Hepatitis B and C viruses cause almost 80 percent of all liver cancer in the world. Epstein-Barr virus can also be carcinogenic, causing cancer of the lymphatic system. Human immunodeficiency virus (HIV) or a type of herpesvirus can lead to rare cancers of the lymphatic and circulatory systems. Helicobacter pylori, a bacterium associated with stomach ulcers, likely causes cancer of the stomach.
In developing countries, parasitic organisms are major carcinogens. In parts of Africa, China, and southern Asia, infestation with the liver fluke Clonorchis sinensis causes a form of liver cancer. In North Africa, infection with the parasite Schistosoma haematobium causes cancer of the bladder.