Cancer is a disease in which some of the body's cells grow uncontrollably and spread to other parts of the body. Cancer can start almost anywhere in the human body, which is made up of trillions of cells. This abnormal cellular growth can spread to the rest of the body. When this uncontrolled growth happens in solid tissue like an organ, muscle, or bone. Other names for cancer are: malignant tumor or malignant neoplasm.