Portions of the following is provided by the National Cancer Institute

According to the Leukemia and Lymphoma Society, leukemia is a malignant disease (cancer) that originates in a cell in the marrow. It is characterized by the uncontrolled growth of developing marrow cells.

There are two major classifications of leukemia: myelogenous or lymphocytic, which can each be acute or chronic. The terms myelogenous or lymphocytic denote the cell type involved. Thus, four major types of leukemia are: acute or chronic myelogenous leukemia and acute or chronic lymphocytic leukemia.

Acute leukemia is a rapidly progressing disease that results in the accumulation of immature, functionless cells in the marrow and blood. The marrow often can no longer produce enough normal red and white blood cells and platelets.

Anemia, a deficiency of red cells, develops in virtually all leukemia patients. The lack of normal white cells impairs the body's ability to fight infections. A shortage of platelets results in bruising and easy bleeding. Chronic leukemia progresses more slowly and permits greater numbers of more mature, functional cells to be made.

In the United States, about 2,000 children and 27,000 adults are diagnosed each year with leukemia. The incidence of leukemia is more common in men and boys than girls and women, and also more likely to occur in white people than black.

Exposure to high-energy radiation (such as World War II atomic bomb explosions) and intense exposure to low-energy radiation from electromagnetic fields (such as power lines and electric appliances like electric blankets) have been linked to leukemia. Studies are being conducted to further understand this link.

There is also a genetic component to leukemia. Certain genetic conditions predispose people to leukemia. For example, those children with Down Syndrome are more likely than the general population to develop this type of cancer.

Another risk factor for developing leukemia is exposure to certain toxic chemicals, such as benzene and formaldehyde. Also, some of the chemotherapy drugs used to treat other types of cancer may increase a person's risk of getting leukemia. However, this risk is very small when compared with the often-enormous benefits of chemotherapy.