Adult T-cell Leukemia (ATL) is a rare and aggressive form of cancer that affects the T-cells, a type of white blood cell crucial for the immune system. It is primarily caused by the Human T-cell Lymphotropic Virus type I (HTLV-I), a virus that can be transmitted through blood, sexual contact, or from mother to child during breastfeeding. Symptoms may include swollen lymph nodes, skin lesions, and fatigue. Diagnosis of ATL typically involves blood tests, imaging studies, and bone marrow biopsies to assess the presence of abnormal T-cells. Treatment options may include chemotherapy, targeted therapy, and stem cell transplants, but the prognosis can vary significantly among individuals.