Leukemia is cancer of your body's blood-forming tissues, including the bone marrow and lymphatic system. The word "leukemia" means "white blood" in Greek. The disease usually starts in the white blood cells.

Under normal circumstances, your white blood cells are potent infection fighters. These cells normally grow and divide in an orderly, controlled way, as your body needs them. But leukemia disrupts this process.

In leukemia, your bone marrow produces a large number of abnormal white blood cells. They look different from normal blood cells and don't function properly. Eventually, they block production of normal white blood cells, impairing your ability to fight off infection. Leukemia cells also crowd out other types of blood cells produced by the bone marrow, including red blood cells, which carry oxygen to tissues throughout your body, and platelets, which help form blood clots.

Leukemia isn't just a children's disease, as some people think. Leukemia has four main types and many subtypes — and only some of them are common among children. Treatment of leukemia is complex — and it depends on your age and health, the type of leukemia and how far it has spread.

Signs and symptoms for each type of leukemia differ, but common symptoms include:

• Fever or chills
• Persistent fatigue, weakness
• Frequent infections
• Loss of appetite or weight
• Swollen lymph nodes, enlarged liver or spleen
• Easy bleeding or bruising
• Shortness of breath when you're physically active, as while climbing steps
• Tiny red marks in your skin (petechiae)
• Excessive sweating, especially at night
• Bone pain or tenderness

The severity of signs and symptoms depends on the number of abnormal blood cells and where they collect. You may overlook early symptoms of leukemia because they may resemble symptoms of the flu and other common illnesses.

Doctors classify leukemia in two ways.

Speed of progression
The first type of classification is by how fast the leukemia progresses:

• Acute leukemia. In acute leukemia, the abnormal blood cells are immature blood cells (blasts). They can't carry out their normal work, and they multiply rapidly, so the disease worsens quickly. Acute leukemia requires aggressive, timely treatment.
• Chronic leukemia. This type of leukemia involves more mature blood cells. These blood cells replicate or accumulate more slowly and can function normally for a period of time. Some forms of chronic leukemia produce no signs or symptoms and can go unnoticed or undiagnosed for years.

Types of cells affected
The second type of classification is by type of white blood cell affected:

• Lymphocytic leukemia. This type of leukemia affects the lymphoid cells or lymphocytes, which form lymphoid or lymphatic tissue. This tissue is the main component of the immune system and is found in various places throughout your body, including your lymph nodes, spleen and tonsils.
• Myelogenous (MI-uh-loj-uh-nus) leukemia. This type of leukemia affects the myeloid cells. The myeloid cell line includes cells that later develop into red blood cells, white blood cells and platelet-producing cells.

Major types
The major types of leukemia are:

• Acute myelogenous leukemia (AML). This is the most common type of leukemia. It occurs in children and adults. It's also called acute nonlymphocytic leukemia.
• Acute lymphocytic leukemia (ALL). This is the most common type of leukemia in young children. ALL accounts for 80 percent of all childhood leukemias.
• Chronic lymphocytic leukemia (CLL). With CLL, a common adult leukemia, you may feel well for years without treatment. CLL is more common in Jewish people of Russian or Eastern European descent. It almost never affects children.
• Chronic myelogenous leukemia (CML). This type of leukemia mainly affects adults. It's associated with a chromosome abnormality called the Philadelphia chromosome, which creates an abnormal gene called BCR-ABL. The BCR-ABL gene produces an abnormal protein called tyrosine kinase that doctors and researchers believe causes leukemia cells to grow and develop. A person with CML may have few or no symptoms for months or years before entering a phase in which the leukemia cells grow more quickly.
• Other chronic myeloid disorders. Like CML, this group of diseases causes chronic leukemia by the creation of too few or too many myeloid cells. Chronic myeloid disorders include myelodysplastic syndromes and myeloproliferative disorders such as essential thrombocythemia, polycythemia vera and myelofibrosis. These conditions may lead to acute myeloid leukemia.

Other, rarer types of leukemia include hairy cell leukemia and chronic myelomonocytic leukemia.

Causes unclear
Doctors don't understand the exact cause of leukemia. It seems to develop from a combination of genetic and environmental factors.

• Acute leukemia begins with one or a few white blood cells that have a lost or damaged DNA sequence. These cells remain immature in what's known as a blast form, but maintain the ability to multiply. Because they don't mature and then die as normal cells do, they accumulate and begin to interfere with functions of vital organs. Eventually, they overwhelm the production of healthy cells.

• Chronic leukemia involves more mature blood cells. They replicate and accumulate more slowly, so the progression of the disease is slower but it can still be deadly. Experts aren't sure why this process begins.

  Eventually, a shortage of normal white blood cells leads to infection, anemia and excessive bleeding. Too many abnormal white blood cells can impair the function of bone marrow and infiltrate other organs. Death usually results from bleeding or infection.

These factors place you at an increased risk of developing some types of leukemia:

• Cancer therapy. People who've had certain types of chemotherapy and radiation therapy for other cancers have a slightly greater risk of developing certain types of leukemia many years later.
• Genetics. Genetic abnormalities seem to play a role in the development of leukemia. Certain genetic diseases, such as Down syndrome, are associated with increased risk of leukemia.
• Exposure to radiation and certain chemicals. People exposed to very high levels of radiation, such as survivors of an atomic bomb blast or a nuclear reactor accident, have an increased risk of developing leukemia. Exposure to certain chemicals, such as benzene — which is found in unleaded gasoline and tobacco smoke and used by the chemical industry — is also linked to greater risk of some kinds of leukemia.

However, most people with known risk factors don't get leukemia. And many people with leukemia have none of these risk factors.

Persistent fatigue, weight loss, easy bruising, bleeding, swollen lymph glands and fever are possible indications of leukemia. If you have these signs or symptoms, see your doctor to determine the underlying cause.

Doctors often find chronic leukemia in a routine blood test, before symptoms begin. If this happens, or if you or your child has symptoms that suggest leukemia, you may undergo the following diagnostic exams:

• Physical exam. Your doctor will look for physical signs of leukemia, such as pale skin from anemia and swelling of your lymph nodes, liver and spleen.
• Blood tests. By looking at a sample of your blood, your doctor can determine if you have abnormal levels of white blood cells or platelets — which may indicate leukemia.
• Cytogenetic analysis. This test detects changes in the chromosomes, including the presence of the Philadelphia chromosome. It can be done using a regular microscope or a more modern lab technology called fluorescence in situ hybridization (FISH).
• Bone marrow sample. If your doctor suspects leukemia, he or she may refer you to a doctor who specializes in cancer (oncologist) or a doctor who specializes in blood (hematologist). This specialist can use a needle to remove a sample of your bone marrow to look for leukemia cells.

You'll need additional tests to confirm the diagnosis and to determine the type of leukemia and its extent in your body. Certain types of leukemia are classified into stages, indicating the severity of the disease. Staging helps your doctor determine a treatment plan.

Unlike other types of cancer, leukemia isn't a solid tumor that your doctor can surgically remove. The source of the problem is really the bone marrow. But you can't just remove bone marrow because it produces three basic types of cells — white blood cells, red blood cells and platelets.

Treatment for leukemia is complex. It depends on many factors, including your age and overall health, the type of leukemia you have and whether it has spread to other parts of your body.

Therapies used to fight leukemia include:

• Chemotherapy. Chemotherapy is the major form of treatment for leukemia. This treatment uses chemical agents to kill leukemia cells. Depending on the type of leukemia you have, you may receive a single drug or a combination of one or more drugs. These drugs may come in a pill form, or they may be injected directly into a vein.
• Biological therapy. Also known as immunotherapy, biological therapy uses substances that bolster your immune system's response to cancer.
• Kinase inhibitors. For most people with CML, the drug imatinib mesylate (Gleevec) is the first line of therapy. Imatinib mesylate is a type of cancer drug called a kinase inhibitor. It was specifically developed to inhibit the BCR-ABL protein, and it has proved effective in treating the early stages of chronic myelogenous leukemia. Several newer kinase inhibitors are in development.
• Other drug therapy. Arsenic trioxide and all-trans retinoic acid (ATRA) are anti-cancer drugs that doctors can use alone — or in combination with chemotherapy — to treat a certain subtype of AML called promyelocytic leukemia. These drugs cause leukemia cells with a specific gene mutation to mature and die.
• Radiation therapy. Radiation therapy uses X-rays or other high-energy rays to damage leukemia cells and stop their growth. You may receive radiation in one specific area of your body where there is a collection of leukemia cells, or you may receive radiation directed at your whole body.
• Bone marrow transplant. This process replaces your leukemic bone marrow with leukemia-free marrow. In this treatment, you receive high doses of chemotherapy or radiation therapy, which destroys your leukemia-producing blood marrow. This marrow is then replaced by bone marrow from a compatible donor. In some cases, you may also be able to use your own bone marrow for transplant (autologous transplant). This is possible if you go into remission and then save healthy bone marrow for a future transplant, in case the leukemia returns.
• Stem cell transplant. Stem cell transplant is similar to bone marrow transplant except the cells are collected from stem cells that circulate in the bloodstream (peripheral blood). The cells used for transplant can be your own healthy cells (autologous transplant), or they can be collected from a compatible donor (allogeneic transplant). Doctors use this procedure more frequently than bone marrow transplant because of shortened recovery times and possible decreased risk of infection.
• Clinical trials. Some people with leukemia choose to enroll in clinical trials to try out experimental treatments or new combinations of known therapies.
• Supportive care. No matter what kind of cancer therapy you choose, you'll likely need medications to control pain and side effects.

A diagnosis of leukemia can be devastating — especially for the family of a newly diagnosed child. Remember that no matter what your concerns or prognosis, you're not alone. The road ahead may not be easy, but these strategies and resources may make it easier:

• Know what to expect. If you or your child is diagnosed with leukemia, find out everything you can about the type, the stage, the treatment options and their side effects. The more you know, the more active you can be in your own care. In addition to talking with your doctor, seek out information from reliable sources such as the National Cancer Institute, the American Cancer Society, and the Leukemia and Lymphoma Society.
• Be proactive. Although you may feel tired and discouraged, don't let others — including your family or your doctor — make important decisions for you. Take an active role in your treatment.
• Maintain a strong support system. Having a support system and a positive attitude can help you cope with the issues, pain and anxieties that might occur. The concern and understanding of a formal support group or others coping with cancer can be especially helpful. Although support groups aren't for everyone, they can be a good source for practical information. You may also find you develop deep and lasting bonds with people who are going through the same things you are.
• Set reasonable goals. Having goals helps you feel in control and can give you a sense of purpose. But don't choose goals you can't possibly reach. You may not be able to work a 40-hour week, for example, but you may be able to work at least part time. In fact, many people find that continuing to work can be helpful.
• Take time for yourself. Eating well, relaxing and getting enough rest can help combat the stress and fatigue of cancer. Also, plan ahead for the downtimes when you may need to rest more or limit what you do.
• Stay active. Having cancer doesn't mean you have to stop doing the things you enjoy or normally do. For the most part, if you feel well enough to do something, go ahead and do it. It's important to stay involved as much as you can.