A carcinogen is any substance that is directly linked to the development of any type of cancer. Carcinogens can cause cancer in many ways, such as changing the cellular genome or interfering with metabolism. Among the best known carcinogens are inhaled asbestos, dioxin and tobacco smoke. However, not all carcinogens are artificial. Alfatoxin, which is a component of types of fungus, hepatitis B and the human papilloma virus are also linked to certain types of cancer.