Leukemia cells often don't fight infections like normal white blood cells do. Even when the abnormal white blood cells function almost normally, they eventually become so numerous that they interfere with the function of the normal blood cells.

The disease is further divided by the types of blood cells it affects. Myeloid leukemia affects the myeloid cells; lymphocytic leukemia affects the lymphoid cells. The names of the 4 common types of leukemia are acute myeloid leukemia (AML), acute lymphocytic leukemia (ALL), chronic myeloid leukemia (CML), and chronic lymphocytic leukemia (CLL).

AML is slightly more common among men than women. Doctors don't know why. AML is one of the most common types of leukemia in adults. It's less common in children.

In most cases, the cause of leukemia is not known, but certain environmental and genetic factors appear to increase the risk for the disease. These include not only exposure to benzene, but also exposure to high levels of radiation, either through wartime or disaster, or through medical radiation treatment; chemotherapy; smoking; and Down syndrome and other genetic diseases caused by abnormal chromosomes.

It is more likely to occur in young children. Although the cause of ALL isn't known, it occurs more often in developed countries and in families with higher socioeconomic standing. Besides affecting young children, it also affects older adults.

This defect, called the Philadelphia chromosome, occurs only in leukemia cells, not the rest of the cells in the body. In CML, parts of chromosomes 9 and 22 switch places, an exchange called a translocation. Researchers do not know why this occurs. Other chromosome changes are seen in some other types of leukemia, but these changes are not seen in all cases.

Many doctors don't consider acute leukemia to be cured until a person has no signs of disease for at least several years after treatment--and not all doctors agree with even that definition. The goal of treatment for acute leukemia is complete remission: no trace of the disease. Treatment for both chronic and acute leukemia may involve chemotherapy, targeted therapy, biological therapy, radiation therapy or a stem cell transplant. People with an enlarged spleen may need to have their spleen removed. People with a chronic form of leukemia may not need treatment right away. When treatment is needed, it usually can control the disease and symptoms. Chronic leukemia usually can't be cured, but it can often be controlled for long periods of time.