Leukemia is a type of cancer that starts in blood-forming tissue. When a normal blood cell becomes damaged it dies, and the body replaces it with a healthy blood cell. Unlike normal blood cells, the leukemia cells don’t die off when they become old or damaged. This causes them to crowd out normal healthy cells which provide oxygen, control bleeding, and fight infections. All blood cells come from stem cells. There are three types of blood cells; red blood cells, platelets, and white blood cells. Leukemia can be classified as either acute, which is fast growing, or chronic, which grows at a slower rate.