Experts from Memorial Sloan-Kettering Cancer Center talk about leukemia, a type of cancer that causes bone marrow to make abnormal white blood cells. These abnormal cells eventually crowd out normal blood cells in the bone marrow and lymphatic system. People with chronic (early-stage) leukemia often have very mild symptoms and may not require treatment right away. In the acute stage, treatment is needed to combat symptoms such as severe fatigue, infections, and bleeding. Today, many leukemia patients survive or can be cured of the disease.