Leukemia is a type of cancer that happens when cells in your bone marrow become leukemia cells. Over time, these cells crowd out the healthy blood cells in your blood, bone marrow, and other tissues.
There are four main types of leukemia: acute lymphoblastic (ALL), acute myeloid (AML), chronic lymphocytic (CLL), and chronic myeloid (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.