Leukemia is cancer of the blood cells. Most blood cells form in the bone marrow. In leukemia, cancerous blood cells form and crowd out the healthy blood cells in the bone marrow.The type of leukemia depends on the type of blood cell that has become cancerous. For example, acute lymphoblastic leukemia is a cancer of the lymphoblasts (white blood cells that fight infection). White blood cells are the most common type of blood cell to become cancer. But red blood cells (cells that carry oxygen from the lungs to the rest of the body) and platelets (cells that clot the blood) may also become cancer.Leukemia occurs most often in adults older than 55 years, and it is the most common cancer in children younger than 15 years. Leukemia is either acute or chronic. Acute leukemia is a fast-growing cancer that usually gets worse quickly. Chronic leukemia is a slower-growing cancer that gets worse slowly over time. The treatment and prognosis for leukemia depend on the type of blood cell affected and whether the leukemia is acute or chronic. Chemotherapy is often used to treat leukemia. (source : National Cancer Institute).