There are over 4,000 chemicals in cigarettes. 51 of them are known to be carcinogenic. A carcinogen is something that causes cancer. Cancer is a disease that often kills those who have it. There are many types of cancer: breast, lung, larynx, stomach, prostrate, kidney, leukemia (cancer of the blood), etc. In all kinds of cancer, the cells keep dividing and forming new, abnormal cells. These cells are not normal or healthy.