Cancer is a disease in which some of the body’s cells grow uncontrollably and spread to other parts of the body.  It can start almost anywhere in the human body, which is made up of trillions of cells. Normally, human cells grow and multiply (through a process called cell division to form new cells as the body needs them. When cells grow old or become damaged, they die, and new cells take their place.
Cancerous tumors spread into, or invade, nearby tissues and can travel to distant places in the body to form new tumors (a process called metastasis). Cancerous tumors may also be called malignant tumors. Many cancers form solid tumors, but cancers of the blood, such as leukemias, generally do not.