Leukemia is cancer of the white blood cells. White blood cells help your body fight infection. Your blood cells form in your bone marrow. In leukemia, the bone marrow produces abnormal white blood cells. These cells crowd out the healthy blood cells, making it hard for blood to do its work.
There are different types of leukemia, including
Leukemia can develop quickly or slowly. Chronic leukemia grows slowly. In acute leukemia, the cells are very abnormal and their number increases rapidly. Adults can get either type; children with leukemia most often have an acute type. Some leukemias can often be cured. Other types are hard to cure, but you can often control them. Treatments may include chemotherapy, radiation and stem cell transplantation. Even if symptoms disappear, you might need therapy to prevent a relapse.
NIH: National Cancer Institute