What is Leukemia? Leukemia is a cancer of the body's blood-forming tissues, including the bone marrow and the lymphatic system. [Image of the bone marrow producing blood cells] It typically involves the white blood cells. In a healthy body, white blood cells are potent infection fighters. In people with leukemia, the bone marrow produces excessive, abnormal white blood cells that don't function properly, eventually crowding out healthy red cells and platelets. Symptoms Leukemia symptoms are often vague and can easily be mistaken for the flu or other common illnesses. However, unlike a temporary virus, these signs persist and worsen over time: Recurrent fever or chills. Persistent fatigue and physical weakness that doesn't improve with rest. Frequent, severe, or unusual infections. Unexplained weight loss. Swollen lymph nodes (usually painless), an enlarged liver, or an ...