An estimated 4,700 Canadians will learn they have leukemia in 2009, according to the Canadian Cancer Society. Within five years, just under 2,000 of those people will die of their illness.

The five-year survival-rate for all types of leukemia is 51 per cent.

The four most common leukemias are:

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

All types of leukemia develop in a cell in bone marrow. The cell will undergo some kind of change and then multiply. Eventually, the leukemia cells will crowd out normal marrow and white blood cells.

Blood is made up of white blood cells, red blood cells and platelets. They are carried in a fluid called plasma.

White blood cells are a critical part of the body's immune system: they help prevent and fight infection by killing bacteria, viruses and other cells that may invade your body.

Red blood cells carry oxygen throughout your body and platelets help your body control bleeding.

Acute leukemias develop quickly and are aggressive. Signs or symptoms can be similar to other, more common and less severe illnesses. They can include a lack of energy, shortness of breath during physical activity, night sweats, pale skin, excess bleeding, slow healing of cuts, bruising for no apparent reason, tiny red spots under the skin and aches in bones or joints.

In acute lymphoblastic leukemia, immature white blood cells multiply rapidly in bone marrow, crowding out normal cells. The cancer will spread to other organs if untreated. The disease is most common in children between the ages of four and 12, and in adults in old age. But the overall cure rate in children is 85 per cent and 50 per cent in adults.

Toronto Maple Leafs' Jason Blake skates during warmup Oct.9, 2007, a day after revealing he has a rare form of leukemia that is highly treatable. (Adrian Wyld/Canadian Press)Acute myelogenous leukemia is a relatively rare disease, yet it is the commonest form of acute leukemia affecting adults. It features the rapid growth of abnormal white blood cells in the bone marrow, interfering with the production of normal blood cells. Counts for red blood cells, platelets and normal white blood cells drop dramatically. The disease progresses rapidly and can lead to death within weeks or months if untreated.

Chronic leukemias develop far more slowly. People with CLL or CML may not even know they are ill. The disease is often diagnosed after blood tests for something else.

When Toronto Maple Leaf forward Jason Blake revealed in October 2007 that he had chronic myelogenous leukemia (CML), he said the disease would not keep him out of the lineup. And - due to recent advances in the treatment of the disorder - he was right. He has played two full NHL seasons since revealing his condition.

While Blake and the estimated 450 people who are diagnosed with this uncommon form of leukemia each year may never be cured of the disease, there are now drugs available that allow patients to manage the disease.

Two drugs — imatinib and dasatinib — were designed to target the abnormal gene that causes CML and other types of leukemia from working.

"[Imatinib] has a very high response rate," Dr. Mark Minden, a leukemia specialist at Princess Margaret Hospital in Toronto, told CBC News. "Over 90 per cent of patients will have their counts return to normal and over a period of time the leukemic cells disappear and normal cells come back. Most patients will have a long-term remission from this drug and can function totally normally with it."

Targeted drugs have only been a treatment option for ALL, AML and CML for less than a decade. Since they became available, the survival rate for the disease has been steadily improving.

Research shows that people on imatinib and dasatinib tend to have a high rate of remission and lower rates of recurrence of the disease after five years. Minden estimates that the long-term survival rate is approximately 80 per cent.

"We have patients in all walks of life doing their normal jobs," Minden said. "We have patients who are high-performance athletes who are also continuing to function very normally with this drug. It doesn't have the same side-effects as the more toxic chemotherapies we use that are less targeted."

Meanwhile, research has also suggested that imatinib — commercially known as Gleevac — could be a promising treatment for children with one of the most aggressive forms of leukemia. Philadelphia chromosome-positive acute lymphoblastic leukemia or Ph+ALL, accounts for about three to five per cent of acute lymphoblastic leukemia cases in children, and one in five cases in adults are this type.

The disease is currently treated with marrow transplants and chemotherapy, but relapse rates are high.

By combining chemotherapy with imatinib, three-year survival rates improved from 35 per cent before the targeted drug was used to 87 per cent afterwards, for those who took it for more than 280 days.

The researchers say more study is needed to see if the numbers hold up for five-year survival rates. The results suggest that this kind of therapy could mean the end of bone marrow transplants and chemotherapy for treating this kind of cancer.

The drug is expensive: it can cost $80,000 per patient for each year of treatment.

Other leukemia treatments are similar to treatments for other cancers. They can include:

• Chemotherapy: either by pill or intravenously. Chemotherapy drugs combat the ability of cancer cells to grow and spread. They can also harm healthy cells.
• Stem cell transplant: intense chemotherapy can destroy healthy bone marrow along with cancer cells. When this happens, bone marrow may be replaced through stem cell transplants - either from a closely matched donor or from the patient (healthy stem cells are harvested before treatment begins).
• Radiation therapy: daily doses of radiation beams are used to damage cells in the beam's path.

There is no single known cause of leukemia, but there are several risk factors. Among them are:

• Age. The older you are, the higher your risk.
• Previous treatment with radiation or chemotherapy.
• Exposure to high levels of radiation, such as nuclear fallout.
• Exposure to chemicals such as benzene.
• Smoking.
• Family history.
• Having a blood disorder, such as myelodysplastic syndrome.

In some cases, people develop leukemia without any of the above risk factors.