Leukemia is a type of cancer that happens when cells in your bone marrow grow abnormally and out of control. These cells can spread to other parts of your body.
There are four main types of leukemia: acute lymphoblastic (ALL), acute myelogenous (AML), chronic lymphocytic (CLL), and chronic myelogenous (CML). The acute types can get worse quickly and need to be treated right away. The chronic types get worse slowly and may not need to be treated until you have symptoms.