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Leukemia is cancer of blood-forming tissue such as bone marrow. Types of leukemia are grouped by the type of cell affected and by the rate of cell growth. Leukemia is either acute or chronic.

Myelodysplastic Syndrome (MDS) is a condition in which the bone marrow does not function normally and therefore does not produce enough normal blood cells. The blood cells affected are white blood cells, red blood cells and platelets. Some cases of MDS may, over time, progress to acute leukemia. MDS is most often found in patients nearing their 60s and 70s. However, there are always exceptions.

The specific cause of leukemia is still not known. Scientists suspect that viral, genetic, environmental or immunologic factors may be involved.

Some viruses cause leukemia in animals. But in humans, viruses cause only one rare type of leukemia. Even if a virus is involved, leukemia is not contagious. It can not spread from one person to another. There is no increased occurrence of leukemia among people such as friends, family and caregivers who have close contact with leukemia patients.

There may be a genetic predisposition to leukemia. There are rare families where people born with chromosome damage may have genes that increase their chances of developing leukemia.

Environmental factors, such as high-dose radiation and exposure to certain toxic chemicals, have been directly linked to leukemia. But this has been true only in extreme cases, such as atomic bomb survivors in Nagasaki and Hiroshima or industrial workers exposed to benzene. Exposure to ordinary x-rays, like chest x-rays, is not believed to be dangerous.

People with immune-system deficiencies appear to be at greater risk for cancer because of the body’s decreased ability to resist foreign cells. There is evidence that patients treated for other types of cancer with some types of chemotherapy and/or high-dose radiation therapy may later develop leukemia.

All of these factors may explain why a small number of people develop leukemia. But, among most people, the cause of leukemia is not known.

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