Hemophagocytic lymphohistiocytosis (HLH) is a rare disease that usually occurs in infants and young children. It may also occur in adults. Children usually inherit the disease. In adults, many different conditions, including infections and cancer, can cause HLH. If you have HLH, your body's defense system, called your immune system, does not work normally. Certain white blood cells — histiocytes and lymphocytes — attack your other blood cells.