Leukemia is a cancer of the blood, or bone marrow. Bone marrow produces the blood cells. When there occurs a problem in the production of blood cells and the DNA of immature white blood cells gets damaged, leukemia occurs. It can develop both quickly or get worsen over time. It is more likely to affect the people above the age of 55 years, but is also common among children under age 15 years. Leukemia can be fatal, but there are ways of treating and controlling the disease.