Leukemias are cancers of the body’s blood-forming cells. They affect the body’s blood making system, including bone marrow and the lymphatic system.

Bone marrow is the soft, inner component of bones. All forms of blood cells are produced in the bone marrow including: 

• White blood cells (fight infection)
  
  
• Red blood cells (carry oxygen to tissues all over the body)
  
  
• Platelets (help develop blood clots and control bleeding)

In patients with leukemia, the bone marrow manufactures a large number of abnormal white blood cells. Over time they begin to increase, crowding out normal white blood cells, red blood cells and platelets. This results in anemia, bleeding disorders and a decreased ability to fight off infection.

Leukemia develops in the bone marrow but typically spreads quickly into the blood. Eventually it spreads further into the lymph nodes, the liver, the covering of the spinal cord and brain, spinal fluid, the spleen and other organs. Without successful treatment, the disease is usually fatal.

There are many types and subtypes of leukemia. Most forms of leukemia can be classified into four main types. These include:

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

In addition to the four main types, there are also a number of rare types of leukemia.

The exact cause of leukemia has not been identified. Researchers have gained a greater understanding of how specific changes in DNA can cause cells to develop into leukemia. A number of risk factors have been identified, including smoking and exposure to high levels of radiation and certain chemicals. 

Symptoms of leukemia vary depending on the exact type of the disease and where it has spread in the body. In chronic leukemia, the symptoms may not appear for a long time. In acute leukemia, the symptoms usually develop quickly and rapidly become more severe.

The most common symptoms include fatigue, weakness and weight loss.

Leukemia may be first diagnosed by routine blood tests. However, additional tests that may be necessary include bone marrow tests, biopsy and lumbar puncture. The treatment for leukemia depends upon the type and severity of the disease. It may include chemotherapy, radiation therapy, biological therapy and bone marrow transplantation. Leukemia patients usually receive a combination of these therapies.

The American Cancer Society (ACS) estimates that there will be more than 44,000 new cases of leukemia diagnosed in the United States in 2007. The disease develops 10 times more often in adults than in children and is more common in men than women. The ACS estimates there will be slightly more cases of acute forms of leukemia than chronic leukemias.

Survival rates vary by type of leukemia. In 2007, almost 22,000 deaths will occur from leukemia, according to the ACS. Five-year survival rates vary from 20 percent for people with AML to 74 percent for patients with CLL.