Adult T-cell leukemia/lymphoma (ATLL) is a type of cancer that affects the T-cells, a kind of white blood cell important for the immune system. It is caused by the human T-lymphotropic virus type I (HTLV-I), which is primarily transmitted through sexual contact, blood transfusions, or from mother to child during breastfeeding. ATLL can manifest in various forms, including acute, chronic, and smoldering types, each with different symptoms and progression rates. Symptoms of ATLL may include swollen lymph nodes, skin lesions, and fatigue. Diagnosis typically involves blood tests, imaging studies, and sometimes a biopsy of affected tissues. Treatment options may include chemotherapy, antiviral medications, and stem cell transplants, depending on the specific type and stage of the disease.