Cancer is a disease where cells in your body grow out of control and invade different tissues in your body. Normally, cells grow and divide to form new cells as the body needs them. As cells grow old, they die and the new cells take their place. If new cells form when the body does not need them, or old cells don’t die when they should, these extra cells can form a mass of tissue called a tumor. Not all cancers form tumors. For example, leukemia which is cancer of the blood-forming tissue does not form a solid tumor.