When it comes to fever, let’s first look at what is considered a fever? The body temperature of a healthy person is regulated by the thermoregulatory center and remains within a relatively constant range. The normal body temperature is as follows: oral temperature (or sublingual temperature) ranges from 36.3℃ to 37.2℃, rectal temperature is about 0.3℃ to 0.5℃ higher than oral temperature, and axillary temperature is about 0.2℃ to 0.4℃ lower than oral temperature. There are many causes of fever, which can be broadly divided into two categories: infectious fever and non-infectious fever. Infectious fever (also known as “inflammation”): Infectious fever is caused by pathogens such as bacteria, viruses, mycoplasma, fungi, parasites, rickettsiae and spirochetes. The invasion of various pathogens into the body may cause fever and corresponding diseases. Therefore, infectious diseases account for 50-60% of the causes of fever non-infectious fever, that is, fever that is not caused by pathogens such as bacterial infection but still causes fever is non-infectious fever: fever caused by causes other than pathogenesis is non-infectious fever. There are many common etiologies, but the probability of each occurring is not very high, for example: malignant tumors, some patients with malignant tumors are prone to fever, such as leukemia, malignant lymphoma, multiple myeloma, tumors of other sites such as lung cancer, ovarian cancer, etc.; rheumatic diseases: for example, systemic lupus erythematosus, rheumatoid arthritis, etc.; injuries to body tissues such as area burns, trauma, and after major surgery can also cause Fever. Laboratory tests can supplement the history and physical examination, especially for patients with fever as the main symptom and lack of other symptoms and signs. Routine blood tests are convenient, quick and will provide many clues to the diagnosis, such as: Most of the above-mentioned causes of fever together will lead to abnormal white blood cell counts. If neutrophils are elevated, it suggests a bacterial infection, especially a septic infection; it is also seen in certain viral infections such as epidemic hemorrhagic fever. Extremely elevated leukocytes are most often seen in leukemia and leukemia-like reactions. If primitive cells are found during classification, leukemia is highly suspected and further hospitalization is needed to avoid delays. Normal or decreased white blood cell counts are most often seen in viral infections and certain bacterial infections (e.g., typhoid fever, tuberculosis). Other rare conditions such as eosinophilia are seen in parasitic diseases, allergic diseases (allergies, bronchial asthma, some skin diseases), while the absence of eosinophils supports typhoid fever. If there is an increase in heterogeneous lymphocytes in the routine blood classification, it is most likely due to infectious mononucleosis. If whole blood cytopenia is present: it is most often seen in severe infections, aplastic anemia, acute leukemia, malignant histiocytosis, systemic blood-borne tuberculosis, bone marrow metastases from cancer, AIDS, etc., suggesting severe disease. Since many kinds of diseases are capable of causing abnormalities in white blood cells, it is necessary to perform routine blood tests when fever is present to identify the cause as soon as possible to prevent delays and provide timely treatment.