Leukemia is a cancer of the blood and bone marrow that affects white blood cells, red blood cells, and platelets. It has no known single cause but can be caused by factors like radiation, chemicals, and genetic disorders. Leukemia is not contagious unless blood is mixed between an infected and uninfected person. Both chronic and acute forms exist, with signs including fatigue, infections, and abnormal blood test results. Diagnosis involves blood tests, biopsies, and imaging exams. Treatment aims to destroy the cancer cells and may include chemotherapy, stem cell transplants, targeted therapy, and blood transfusions.