Wait, small point of order, leukemia does originate in the bone marrow, which is the spongy tissue inside the bones where blood cells are produced. However, it's considered a blood cancer because it affects the blood cells - specifically, the abnormal white blood cells produced in the bone marrow eventually enter the bloodstream, causing problems throughout the body. So in that sense, it's both a bone marrow disease and a blood disease, but it's usually classified as a blood cancer because of the effect on the blood.