Leukaemia is a cancer that starts in blood-forming tissue such as the bone marrow, which makes blood cells (red blood cells that carry oxygen around the body, white blood cells that fight disease and infection, platelets that help to stop bleeding when it starts). In people with leukaemia, the bone marrow produces large numbers of abnormal white blood cells and not enough normal red blood cells. Leukaemia cases represent less than 4% of all cancer cases in adults but are the most common form of cancer in children. There are different types of leukaemia (e.g. acute, chornic, myeloid and lymphoid leukaemia).