While some animal cancers have been found to be contagious, no form of human cancer has yet been found contagious. Certain contagious illnesses can, however, cause cancer in humans. These illnesses are not passed through cancerous cells but through the viruses causing the cancer. Additionally, some behaviors are thought to be contagious, or at least may cause exposure to cancer causing agents. In this case, the cancer cells are not contagious, but the behaviors causing cancer may be more likely to occur in family groups, like smoking.

Some viruses have a direct link to certain cancers. For example, some forms of human papillomavirus (HPV) are now indicated as a primary cause of all cervical cancers, and may as well cause penile cancer. There are only a few types of human papillomavirus that cause such cancer, and not everyone who has the virus will get cervical cancer. All types of HPV that can cause cervical cancer are sexually transmitted.

Another sexually transmitted illness that is indicated in a form of cancer is human herpes virus 8 (HHV8). It has been linked to the development of Kaposi’s sarcoma and is almost always incorrectly identified as being caused by AIDS. This misunderstanding occurs because those with HIV and AIDS are at a much greater risk for contracting HHV8.

Certain behaviors may predispose people toward certain types of cancer and disease. For example, those who smoke not only run the risk of contracting lung cancer, but also may hazard this risk for others around whom they smoke. Additionally, children of parents who smoke are more likely to smoke themselves. So in an nontraditional sense this behavior can be thought to be “contagious.”

Alcoholism is another form of indirectly “contagious” behavior that increases risk for both stomach and liver cancer. Children of alcoholics are more likely to become alcoholics. While this may be in part behavioral, there may also be genetic factors that predispose people toward addiction. In these cases behavior passed from one generation to the next can increase risk of certain forms of cancer.

In the traditional sense of "contagious," most forms of cancer cannot be passed to another person. However, the recent discovery of HPV’s role in cervical cancer raises questions about whether other cancers may be the result of exposure to certain viruses or bacteria. Scientists continue to research this area in the hope of finding more ways to both cure and prevent cancer.