Leukemia is a cancer of the blood cells. Leukemia begins in a cell in the bone marrow. The cell undergoes a change and becomes a type of leukemia cell. Once the marrow cell undergoes a leukemic change, the leukemia cells may grow and survive better than normal cells. Over time, the leukemia cells crowd out or suppress the development of normal cells. The rate at which leukemia progresses and how the cells replace the normal blood and marrow cells are different with each type of leukemia.
It is the most common type of blood cancer and affects 10 times as many adults as children. Most people diagnosed with leukemia are over 50 years old.