Origins of cancer

Cell division or cell proliferation is a physiological process that occurs in almost all tissues and under certain circumstances. Normally the balance between division and programmed cell death is rigorously regulated to ensure the integrity of organs and tissues. Mutations in DNA that lead to cancer disrupt these orderly processes. 

The uncontrolled and often rapid proliferation of cells can lead to either a benign tumor or a malignant tumor (cancer). Benign tumors do not spread to other parts of the body or invade other tissues, and they are rarely a threat to life unless they extrinsically compress vital structures. Malignant tumors can invade other organs, spread to distant locations (metastasize) and become life-threatening.

A few types of cancer in animals have been found to be contagious.