Leukemia is a type of cancer that affects the blood-forming tissues, primarily the bone marrow, leading to the uncontrolled production of abnormal white blood cells. These abnormal cells crowd out healthy blood cells, impairing the body’s ability to fight infection, carry oxygen, and control bleeding. Leukemia is classified as acute or chronic depending on how quickly it progresses, and as lymphocytic or myeloid based on the type of white blood cell involved. Symptoms may include fatigue, frequent infections, easy bruising or bleeding, pale skin, bone pain, swollen lymph nodes, and unexplained weight loss. Diagnosis is typically made through blood tests and bone marrow examination, and the disease can vary widely in severity and course depending on the specific type.