Cancer is not one disease, but many diseases that occur in different areas of the body. Each type of cancer is characterized by the uncontrolled growth of cells. Under normal conditions, cell reproduction is carefully controlled by the body. However, these controls can malfunction, resulting in abnormal cell growth and the development of a lump, mass, or tumor. Some cancers involving the blood and blood-forming organs do not form tumors but circulate through other tissues where they grow.