Leukemia is the cancer of blood-forming tissue. This includes bone marrow, white blood cells, red blood cells, etc. Types of leukemia are grouped by the types of cells that are affected and by the rate that the cell growth. Leukemia is either acute or chronic. Acute leukemia involves an overgrowth of immature blood cells. This condition is life threatening, because there are not enough mature blood cells to prevent anemia, infection, and bleeding. A diagnosis of acute leukemia is made when there are 20 % or more immature cells in the bone marrow. Myelodysplastic Syndrome is a condition where the bone marrow does not function normally. This causes the body to not produce enough normal blood cells. The most common blood cells affected are white blood cells, red blood cells and platelets. Some cases of MDS over time, progress to acute leukemia. Chronic leukemia involves an overgrowth of mature blood cells. People with chronic leukemia usually have enough mature blood cells to prevent serious bleeding and infection. .
The specific cause of leukemia is still not known. Most people suspect that viral, genetic, environmental, or immunologic factors are to blame. Some viruses cause leukemia in animals, but in humans, viruses only cause one rare type of leukemia. Even if a virus is involved, leukemia is not contagious. It cannot spread from one person to another. There may be a genetic links 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 linked to leukemia. This has been true only in extreme cases. Some cases are atomic bomb survivors in Nagasaki and Hiroshima. People with immune-system deficiencies are at a greater risk to cancer because of the body's decreased ability to resist foreign cells. There is evidence that patients treated for other types of cancer with chemotherapy may also later develop leukemia.