Leukemia is cancer of the white blood cells that form in the bone marrow. In leukemia, the bone marrow produces abnormal white blood cells that crowd out healthy blood cells, making it difficult for blood to function properly. There are two main types of leukemia - acute leukemia, which progresses quickly, and chronic leukemia, which progresses more slowly. The cause of leukemia can include genetic predisposition, exposure to radiation or carcinogenic substances, and sometimes exposure to chemotherapy for another cancer.