Leukemia is a cancer of the white blood cells, which are the cells that help fight infection. The disease develops in the bone marrow. The bone marrow is the soft, spongy center of bones and is where blood cells are produced. 

There are different kinds of leukemia. They are distinguished by how long it takes for the disease to progress (acute or chronic) and the type of white blood cell involved (lymphocyte or myelocyte). Leukemia is the most common form of childhood cancer.