Leukemia is cancer of the blood cells.  Blood cells are formed in bone marrow, which is the soft tissue inside the bone.  These cells include white blood cells, red blood cells and platelets, which are helpful in the clotting blood.  In a normal healthy body, this is exactly what happens.

When a person has leukemia, abnormal blood cells are created in the bone marrow and as is most common with leukemia, abnormal white blood cells are created.  White blood cells are the cells that are responsible for fighting infection in the body. However, the abnormal cells produced in leukemia patients do not function properly or in the same fashion as normal white blood cells. Instead they continue to grow and divide, and the result is the eventual over-crowding of the normal blood cells which means it becomes difficult for the body to control bleeding, fight infection and transport oxygen to all of its various parts.