Carcinogen

A substance or agent causing cancer

Exposure to one or more carcinogens, including certain chemicals, radiation, and certain viruses, can initiate cancer under conditions not completely understood. Some people have a genetic tendency to develop cancer when exposed to a specific carcinogen or combination of carcinogens. Repeated local injury or irritation to a part of the body can be carcinogenic. Identifying and eliminating carcinogens in time can reduce the incidence of cancer.

http://www.merriam-webster.com/dictionary/carcinogen