Viruses, chemicals and co-carcinogenesis.

The etiology of cancers appears to be complex and multifactorial. Peyton Rous and others demonstrated the process of co-carcinogenesis by exposing rabbits to a virus and tars. Epidemiologists have proposed virus-chemical interactions to cause several cancers. For example, one might propose that the etiology of cervical cancer results from a complex interplay between oncogenic viruses and cervical tar exposures through tar-based vaginal douching, cigarette smoking, and/or long-term cooking over wood-burning stoves in poorly ventilated kitchens. Hepatocellular carcinoma may result from the joint effects of viruses and hepatotoxic chemical carcinogens. Kaposi's sarcoma might happen following reciprocal actions of human herpes virus-8 infection, immunosuppression, and chemical exposures, such as nitrite radicals and alumino-silicates. Use of Koch's postulates will not help one prove or disprove a multifactorial causation of disease; new criteria are needed. Delineating the web of causation may lead to additional strategies for prevention and treatment of several cancers.