Question: What is (are) Leukemia ?
Answer: Cancer begins in cells, which make up the blood and other tissues. Normally, cells grow, divide, and produce more cells as needed to keep the body healthy and functioning properly. Sometimes, however, the process of creating a new cell goes wrong -- cells become abnormal and form more cells in an uncontrolled way.
Source: NIHSeniorHealth

Question: What is (are) Leukemia ?
Answer: Leukemia is a cancer of the blood cells. It usually begins in the bone marrow where blood cells are formed. In leukemia, the bone marrow produces abnormal white blood cells. Over time, as the number of abnormal white blood cells builds up in the blood, they crowd out healthy blood cells. This makes it difficult for the blood to carry out its normal functions.
Source: NIHSeniorHealth