1. Cancer is a group of many related diseases that begin in cells, the body’s basic building blocks. To understand cancer, it is helpful to know what happens when normal cells become cancerous.
  2. The body is made up of many types of cells. Normally, cells grow and divide to produce more cells as they are needed to keep the body healthy.
  3. Sometimes, this orderly process goes wrong. New cells form when the body does not need them, and old cells do not die when they should.
  4. Benign tumors are not cancer. They can often be removed and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body. Most important, benign tumors are rarely a threat to life.
  5. Malignant tumors are cancer. Cells in malignant tumors are abnormal and divide without control or order. Cancer cells invade and destroy the tissue around them. Cancer cells can also break away from a malignant tumor and enter the bloodstream or lymphatic system.