Every human being has millions of damaged DNA cells created in body every day. It could be from smoking, drinking alcohol, indigestion, inflammation, stress etc.
But every human being does not develop cancer tumors because the immune system can recognize cancer cells as abnormal, and kill them.
Some human being’s body loses that ability to recognize abnormal cells, for whatever reason. Then cancer takes hold. Cancer cells grow faster than normal cells. And they spread to other parts in body.