Leukemia is several different diseases. It is classified according to how quickly the abnormal changes happen and by the type of blood cell that is affected.

• Acute leukemia gets worse quickly, with fast multiplication of abnormal, immature blood cells called blasts.
• Chronic leukemia worsens gradually. Abnormal cells are present, but they are more mature than they are in acute leukemia and can carry out at least some of their functions. However, they do not fight infection as well as normal white blood cells do. Also, they tend to live much longer than normal white blood cells, which results in an abnormal accumulation of cells.
• Lymphocytic leukemia affects the white blood cells called lymphocytes, which control the body's immune response by finding and destroying foreign substances, such as viruses.
• Myelogenous leukemia affects other kinds of white blood cells in the bone marrow, called granulocytes or monocytes. These help protect the body against bacteria and infections.

Overall a person diagnosed with leukemia usually has one of four main types:

• Acute lymphocytic leukemia (ALL)
• Acute myelogenous leukemia (AML)
• Chronic lymphocytic leukemia (CLL)
• Chronic myelogenous leukemia (CML)

Another less common type is called hairy cell leukemia, a chronic condition in which the lymphocytic cells develop projections that look like tiny hairs.

An estimated 35,000 people are diagnosed with the disease each year. Leukemia is diagnosed 10 times as often in adults than children. The average age of people who develop CLL is about 70 years, and CML about 50 years. Hairy cell leukemia is most common in people who are between the ages of 50 and 60. Only about 2 percent of chronic leukemia patients are children.

The acute forms of leukemia affect both adults and children. The average age of adults who develop AML is 65, and those who develop ALL, older than 70. ALL is much more common in young children, usually developing before age 10. It is responsible for the vast majority of the roughly 2,300 cases of childhood leukemia diagnosed each year.