Acute lymphoblastic leukemia (ALL) is a type of cancer that affects the blood and bone marrow. This disease arises when lymphoblasts, which are the immature forms of white blood cells called lymphocytes, develop genetic mutations. As a result of these mutations, lymphoblasts begin to multiply uncontrollably and rapidly in the bone marrow and blood. The term "acute" indicates that the disease develops suddenly, progresses rapidly if left untreated, and can be fatal within a few months.