Leukemia means a cancer of the blood forming system. This is in the bone marrow - the soft inner part of your bones.

Most types of leukemia cause abnormal white blood cells to be made in the bone marrow.  These cells then get into the blood stream and circulate around the body.  They do not work normally and so do not give you the protection from infection that white blood cells should.

There are several types of leukemia that can develop in adults and children, but two main groups:
 

Juvenile myelomonocytic leukemia (JMML) is a very rare type of childhood leukemia. It is also sometimes called juvenile chronic myelogenous leukemia (JCML).

JMML is most common in children under four years of age. As with many types of cancers, we do not yet know the cause of JMML.  Children with a genetic disorder called neurofibromatosis 1 (NF1) are more at risk of developing JMML.  But this only accounts for about 1 in 10 cases.

In order to understand why leukemia affects you the way it does, it helps to understand how blood cells are normally produced and what they do. Normally, blood cells are produced in the bone marrow.  They are made in a controlled way, when your body needs them. All blood cells start as the same type of cell, called a stem cell.   Stem cells then develop into either a myeloid stem cell or a lymphoid stem cell. Myeloid stem cells eventually develop into red blood cells, platelets and a type of white blood cell called granulocytes. The type of leukemia you have tells you which type of white blood cell has become cancerous. If myeloid cells become cancerous the type of leukemia is known as a myeloid leukemia. JMML is a type of myeloid leukemia.

As the leukemia cells multiply in the bone marrow, fewer normal blood cells are made.  If there are not enough normal blood cells the body cannot function normally. This can cause quite severe symptoms in children with JMML including

The treatment for JMML is usually a stem cell transplant.  This is where doctors replace the damaged stem cells with healthy ones taken from a donor, often a brother or sister.  At the moment this is the only form of treatment that is known to be successful, although it is only suitable for some children. Doctors and scientists are trying to find a more effective treatment for this disease as sadly, it remains difficult to cure.