Low red blood cells, white blood cells, platelets and also fever may be dengue fever, brucellosis, hematologic diseases and so on. 1. Dengue fever: it is an acute infectious disease caused by dengue virus. Patients have sudden onset of illness, mainly with symptoms such as generalized pain, fatigue, fever, rash, hemorrhage, enlarged lymph nodes and leukopenia. When the patient is an acute hemorrhage, it may also lead to reduction of red blood cells and platelets. 2. Brucellosis: it is mainly an acute bacterial infection caused by contact with diseased cattle and sheep, and patients usually have symptoms such as fever, excessive sweating, enlarged lymph nodes, and varying degrees of leukocyte and platelet decline. 3. Blood system diseases: such as aplastic anemia, megaloblastic anemia and other types of anemia, granulocyte deficiency, leukemia-like reaction and other granulocyte diseases, as well as lymphoblastic leukemia, etc., which may cause the patient to have fever, symptoms of lowering of red blood cells, white blood cells and platelets. It is recommended that patients seek timely medical attention to identify the causes and receive appropriate treatment with the help of a doctor.