Infectious diseases including infectious diseases often take fever as the first symptom, when encountering febrile patients, especially critically ill patients, or group fever cases because of unknown diagnosis and failure to provide effective anti-pathogenic treatment, the consequences may delay rescue treatment on the one hand, making the patient’s condition worse or even death, on the other hand, for some infectious diseases, if early diagnosis and early treatment are not achieved may lead to the spread of infectious diseases Therefore, the diagnosis of fever is particularly important. Common pathogens of infectious fever can include viruses, bacteria, fungi, chlamydia, mycoplasma, rickettsia, spirochetes, parasites, etc. The clinical manifestations caused by different pathogens vary, and the clinical manifestations caused by the same pathogen in different individuals (hosts) are not the same, so we need to screen and exclude them one by one to obtain the correct diagnosis. I. Diagnostic points of viral fever (1) Usually self-limiting fever, with a fever duration of less than 2 weeks. (2) The majority of peripheral blood WBC is low, the proportion of lymph in the classification is increased, and a few patients may have heterogeneous gonorrhea (3) Viral rash and enlargement of lymph nodes may appear (4) Multiple organs or tissues may be involved, causing multi-system damage (5) The onset of the disease seems to be more frequent in children, young adults and middle-aged people, and less frequent in the elderly (6) In hosts with immune deficiency, viral fever is prolonged and severe, such as CMV (7) Fever due to some newly emerged or unexplained viral infections, such as SARS, avian influenza, viral hemorrhagic fever (dengue fever), is rapidly changing. (2) Diagnostic points of bacterial fever (1) G+ bacterial infection and G-. Bacterial infections clinical differences Pathogenic objects, pre-morbid health status, route of infection, clinical manifestations, peripheral blood picture (2) the difference between systemic infection and local (focal) infection. The former mainly includes typhoid fever, sepsis, endocarditis, tuberculosis, etc. The latter infection is more insidious and is sometimes overlooked. (3) The difference between common bacterial and specific bacterial infections. Tuberculosis fever especially extrapulmonary tuberculosis, central nervous system infection (4) Fever due to conditionally pathogenic bacteria or opportunistic infections, nosocomial infections, and duel infections (5) Susceptibility factors for fungal infections. Broad-spectrum antibiotics, immunosuppressive agents, host living environment, clinical manifestations including superficial and deep infections. (3) Other rare pathogenic infections (1) Rickettsial disease: such as typhus, scrub typhus, Q fever, etc., attention to human granulocytic anaplasmosis. (2) Chlamydia mycoplasma infections: parrot fever, Mycoplasma pneumoniae pneumonia. (3) Spirochete infections: such as leptospirosis, regression fever, rat bite fever, etc. (4) Parasitic diseases: Pneumocystis carinii, acute schistosomiasis, malaria, toxoplasmosis, etc. (5) Nocardia. (1) Streptococcal infection in pigs (2) Cat scratch fever (3) Yersinia pestis V. Brief description of clinical cases (1) Hepatitis A combined with CMV infection (2) A group fever occurred in a certain place Based on: the object of fever, the speed of transmission, clinical manifestations including fever duration, pharyngeal isthmus, tonsillar erythema, peripheral blood picture, response to antibiotic treatment. Consider the diagnosis: adenovirus infection is likely (3) A male patient, 26 years old, with persistent high fever for one week, ineffective in out-of-hospital fluids and anti-infection, admitted to the hospital with chest radiographs suggestive of pneumonia, but difficult to explain his clinical course by pulmonary infection and poor response to antibiotic therapy, the day after admission, sudden onset of respiratory distress, dramatic drop in WBC and rapid deterioration of general condition, timely thought of certain specific pathogens of infection, and the patient was finally diagnosed with avian influenza. (4) A patient with fever, headache, and elevated blood count, who had a history of “syncope” several times, was diagnosed with renal syndrome hemorrhagic fever when the diagnosis of “rheumatoid encephalitis” was corrected in time to prepare for lumbar puncture. (5) Tuberculosis infection ①A male patient, 22 years old, had persistent high fever for 3 weeks, antibiotic treatment was ineffective, after admission to the hospital, careful physical examination and laboratory tests including biochemistry, serology, various cultures, bone marrow examination did not have special findings, the diagnosis was unknown for a while, later examination revealed that the patient had oral-genital ulcers, the clinical diagnosis was “leukocephalus”. Although the hormonal treatment was effective, the infection did not seem to be completely ruled out? The clinical manifestations of the patient seemed not to be explained by leukoaraiosis, but later the lesion was found on chest CT and anti-TB treatment was given in time to avoid misdiagnosis and mismanagement. ②Tuberculous meningitis Unexplained prolonged fever without obvious headache and central nervous system symptoms, but difficulty urinating or urinary retention may be a symptom of early disease, and early lumbar puncture was performed to clarify the diagnosis. (3) A male patient, 17 years old, with persistent high fever and “Staphylococcus aureus” isolated from blood culture was treated with “Stable” anti-infection therapy in an outside hospital, but his temperature did not subside. “He was cured by anti-TB treatment.