Cancer is a disease where abnormal cells divide uncontrollably and invade other tissues. There are over 100 types of cancer. Leukemia is cancer of the blood cells that starts in the bone marrow. The main types of leukemia are acute lymphocytic, acute myelocytic, chronic lymphocytic, and chronic myelocytic. Symptoms of leukemia include infections, anemia, bruising, weight loss, and bone pain. Risk factors include radiation exposure, smoking, certain chemicals and viruses, and family history. Diagnosis involves physical exams, blood tests, biopsies, and imaging scans.