Leukaemia is the name used to classify a number of cancers which affect the blood cells produced by the body.

The cancers interfere with the body's ability to produce blood cells in the correct quantities.

The most common types are lymphoblastic leukaemia, which affects the "white cells" in the blood, and myeloid leukaemia, which affects the other blood cells.

In both types, the disease can be acute, in which its progress is more rapid, or chronic, which means the disease often does not advance as swiftly.

Although some of the more aggressive acute cases can quickly become life-threatening if left untreated, some chronic patients can survive for years, even without treatment.

What causes leukaemia?

Although some risk factors for the development of leukaemia have been discovered, there is much that is still to be discovered about how adults, and more particularly young children become ill.

There is no current evidence that leukaemia can be inherited - that is, passed from parent to child through the genes.

Although leukaemia itself is not an infectious condition, some scientists believe that some cases may well be triggered by a viral infection, although no real evidence to support this has yet been found.

There is certainly evidence that Hodgkin's lymphoma, a cancer affecting the cells of the lymphoid system which is often compared to leukaemia, may be linked to infection with the Epstein-Barr virus.

However, it is known that exposure to large doses of ionizing radiation, cigarette smoking, and, infrequently, high dose chemotherapy for other cancers, can contribute to the development of leukaemia.

What are the symptoms?

There is no one, clear symptom that points to leukaemia - any of the warning signs could easily be caused by other illnesses or infections.

However, some of those most commonly experienced include:

• Excessive tiredness
• Breathlessness
• Anaemia or paleness
• Joint or bone pain
• Bleeding or excessive bruising
• Persistent infections or fevers
• Discomfort in the abdomen

Doctors advise that anyone who develops persistent symptoms like these should go for a check-up.

A simple blood test can give a strong indication of whether the patient has leukaemia - sufferers will often have very high numbers of white blood cells, and low levels of some red blood cells.

Looking at the blood through a microscope confirms the diagnosis, although a sample of bone marrow - the tissue which produces blood cells - is usually taken.

How is it treated?

The treatment depends very much on the type of leukaemia present and the sort of cells spotted under the microscope.

The aim of treatment is to achieve "remission" - this is a situation in which laboratory tests can find no evidence of cancer cells and the patient has no symptoms.

This is normally attempted through chemotherapy, and the different types and sub-classifications of leukaemia vary widely in their response to the drugs.

Even if remission is achieved, the patient can still relapse, and drug treatment continues for some time to try to prevent this happening.

If relapses occur, the cancer has frequently become far more resistant to the chemotherapy drugs, making it far harder to achieve remission again.

In some cases involving chronic leukaemia, in which the cancer is spreading far more slowly, doctors choose not to give chemotherapy treatment, and the patient may live for years without dying from the disease.

When the bone marrow is so diseased, or highly damaged by chemotherapy, a bone marrow transplant may be recommended.

Bone marrow can come either from the patient, in which case it is removed, treated to kill all the cancer cells then put back, or from a donor.

Very few people can provide an exact bone marrow match, although doctors hope that new techniques under development will cut the need for such an exact match in the future.

Until then, however, leukaemia patients rely on an extensive register of potential bone marrow donors - all volunteers from the general public - who are all willing to give their marrow should it prove to be that elusive match.

This information is not intended as a substitute for qualified medical advice - if in any doubt, consult a doctor.