A disease characterized by a progressive and abnormal accumulation of white blood cells, or leukocytes. Leukemic cells are malignant because they have three characteristics common to all cancers: (1) they exhibit uncontrolled growth that is frequently associated with an inability to mature normally; (2) they arise from a single precursor cell; and (3) they disregard anatomic boundaries and metastasize to organs or tissues where leukocytes are not normally found. The expanding clone of leukemic cells infiltrates organs and tissues, particularly the bloodstream and bone marrow, where they disrupt the production of normal cells. The resulting symptoms include fatigue, pallor, infections, bruising and bleeding, and discomfort caused by enlarged organs. In humans, the term leukemia encompasses more than 20 distinct malignancies.