Leukaemia is a cancer of the white blood cells that are made in the bone marrow. It occurs when normal body cells get out of control, multiply and spread.

When our bodies are working normally, the bone marrow makes all the different blood cells we need to keep healthy. It makes white cells to help us fight infections, red cells to carry oxygen around the body and platelets to help the blood to clot properly.

If you have leukaemia your bone marrow makes too many abnormal white blood cells. These can cause problems in a number of ways:

• there may not be enough normal white cells left to fight infections – and this can make you very ill
• the abnormal cells may take up too much space in the bone marrow, so that the body can't make enough red blood cells and platelets. If you have too few red cells you may become anaemic. If you have too few platelets you may get bleeding problems – from your nose or gums, for instance
• the abnormal cells can get into the bloodstream and travel around the body. They may build up in one place, causing problems such as swelling and pain.