Leukemia is a cancer of the blood characterized by an abnormal increase in white blood cells. There are four main types of leukemia - acute lymphoblastic leukemia, acute myelogenous leukemia, chronic lymphoblastic leukemia, and chronic myelogenous leukemia. Risk factors include radiation, viruses, chemicals like benzene, and genetic predispositions. Symptoms can include fatigue, fever, frequent infections, and easy bleeding. Diagnosis involves blood tests, bone marrow biopsy, and other scans. Treatment may include chemotherapy, radiation therapy, stem cell transplants, and monoclonal antibodies depending on the type of leukemia.