There are many reasons for low fever instead of normal blood test, the common ones may be caused by viral infection, Mycobacterium tuberculosis infection, or lymphoma and other hematologic diseases. 1. When viral infection occurs in human body, such as respiratory syncytial virus, adenovirus, cytomegalovirus, EBV, etc., the routine blood test may not show elevated white blood cell count, but may be accompanied by low fever; 2. When Mycobacterium tuberculosis infection occurs and the patient suffers from tuberculosis, for example, pulmonary tuberculosis, intestinal tuberculosis, etc., there may be no elevated white blood cell count, but there may be low fever, night sweating, emaciation, etc. in the afternoon. 3. In addition, as the disease progresses, lymphoma patients may have group B symptoms, i.e. unexplained low-grade fever, emaciation, night sweating, etc., but at this time, routine blood tests may not show any obvious abnormality. Further examination such as lymph node biopsy is needed to clarify the diagnosis. It is recommended that the patient should consult a doctor in time, complete the relevant examinations to clarify the specific causes of low fever, and then treat the cause.