Community-acquired pneumonia (CAP) is an infectious inflammation of the lung parenchyma that occurs outside the hospital. To this day, CAP remains an important threat to human health, with a mortality rate of 1-5%. With the aging of the society’s population, the increase of immune-compromised hosts, pathogenic changes and the rise of antibiotic resistance rates, SCAP will face more new problems. Therefore, an in-depth understanding of the common pathogens, risk factors, and diagnostic criteria of SCAP, and giving timely and correct treatment are essential to improve the cure rate and reduce the morbidity and mortality rate. 1. Common pathogens of SCAP It is important to know the common pathogens of SCAP for the initial empirical selection of antimicrobial drugs. Streptococcus pneumoniae is the most common pathogen of SCAP, accounting for about 30%. Among them, Streptococcus pneumoniae infection is more likely in men, in patients with non-aspirated pneumonia, in the presence of infectious shock, and in patients who were not on antimicrobials prior to admission. Haemophilus influenzae ranked second, accounting for approximately 6-15% of SCAP. Reports of Legionella pneumophila infections vary widely, up to 40%, and may be related to season of onset and method of examination. In recent years, the incidence of Legionella pneumophila infections has decreased with the widespread use of macrolides and quinolones as antimicrobials. Other common pathogens include Klebsiella pneumoniae, Pseudomonas aeruginosa spp, Staphylococcus aureus, and Cattamora, as well as anaerobic bacteria, fungi, and tuberculosis. In addition, Mycoplasma pneumoniae and Chlamydia pneumoniae and some viruses, such as influenza virus, SARS virus, avian influenza virus, cytomegalovirus can also cause SCAP. 2. Diagnosis of SCAP In the diagnosis of SCAP, it should be clarified whether the patient has CAP, the severity and risk factors of CAP, as well as the diagnosis of the pathogenic bacteria of SCAP. Chest X-ray frontal and lateral chest radiographs should be performed in all patients suspected of CAP, and chest CT examination is feasible for those suspected of left lower lung or mediastinal parietal infection. Chest X-ray is the gold standard for the diagnosis of pneumonia, but it is worth noting that in the early stages of infection, patients with CAP in a dehydrated state and leukopenia may present with relatively normal chest X-rays. Patients with chronic obstructive pulmonary disease and pulmonary alveoli often do not have the typical manifestations of pneumonia. In patients with combined interstitial pulmonary fibrosis, congestive heart failure, and ARDS, pneumonia is difficult to differentiate from the lung shadow of the underlying disease. Therefore, imaging abnormalities should be combined with clinical manifestations and other ancillary findings to make a comprehensive judgment to clarify the presence of CAP. some laboratory tests, such as routine blood, blood electrolytes, blood glucose, liver and kidney function, arterial blood gas analysis or transcutaneous oxygen saturation measurement, and pathogenic tests are essential to evaluate the severity of CAP disease and pathogenic diagnosis. The measurement of blood C-reactive protein and procalcitonin is also helpful in determining the severity and prognosis of the disease. (1) Risk factors of SCAP The morbidity and mortality rate of SCAP is significantly higher than that of general CAP, and the main causes of death are intractable hypoxemia, refractory shock and complications related to pneumonia. In addition, the morbidity and mortality rate of CAP is not only closely related to the severity of the disease, but other risk factors can also increase the morbidity and mortality rate of CAP. These risk factors include age >65 years, residence in a geriatric care home, presence of severe underlying disease, history of CAP hospitalization within the past 1 year, altered mental status, hyperthermia >40°C, presence of aspiration or risk factors predisposing to aspiration, bacteremia, extrapulmonary migrating lesions, hypoproteinemia, metabolic acidosis, and preemptive use of antimicrobial drugs. Clinicians facing CAP patients should first assess their severity and the presence of risk factors, and decide the place and intensity of treatment, which is very important to reduce the medical costs of CAP and improve the cure rate. (2) Pathogenic diagnosis of SCAP Currently, it is believed that sputum pathogenic examination is not required for mild to moderate CAP treated in outpatient clinics, and antibacterial drugs can be selected empirically according to local pathogenic characteristics. In contrast, for SCAP, those who fail initial empirical treatment, and when the pathogenic bacteria may be drug-resistant or rare, sputum, trans-fiberoscopic protective brush or lavage lower respiratory tract sampling, blood, pleural fluid or lung tissue biopsy specimens from CAP patients should be actively taken for smear Gram staining or other special staining microscopy, culture and pathological examination, which can help clarify the pathogenic diagnosis of CAP. Serological examination is helpful in the diagnosis of atypical pathogens, viruses and certain fungal infections. (3) Collection, delivery and laboratory processing of sputum and other specimens Sputum is the most convenient and easily available specimen for pathogenic diagnosis of respiratory tract infection, but it also has the disadvantages of low positivity rate, easy contamination by oropharyngeal colonizing bacteria, and not easy to distinguish pathogenic bacteria from colonizing bacteria. Therefore, mastering the correct method of sputum retention, timely delivery and inoculation, and standardizing laboratory operation methods are essential to improve the positive rate and accuracy of bacterial culture. blood culture should also be actively performed in SCAP patients, and pleural fluid culture should be performed in those with pleural effusion. For sputum, blood, and pleural fluid tests that do not provide a clear pathogenic diagnosis, and when empirical treatment is not effective, or when rare bacterial infections are suspected, invasive methods should be used, such as trans-fibrinoscopic protective brush and lavage sampling, and lung puncture biopsy, to try to clarify the pathogenic bacteria of infection. The determination of righteousness must be clinically integrated. For example, in cases where empirical therapy is effective, the therapeutic agent should not be changed despite the fact that the cultured pathogenic bacteria are outside the antimicrobial spectrum of the chosen antimicrobial agent or resistant to it. And in the case of ineffective empirical treatment, although the cultured pathogenic bacteria are sensitive to the antimicrobial drugs used, it is not certain that it is the real causative organism. 3. Early goal-directed therapy and supportive therapy: SCAP patients often have infection or hypovolemic shock, resulting in insufficient blood supply to vital organs and impaired microcirculation. Early target-directed therapy refers to early active fluid supplementation and shock correction therapy. organ support therapy should be given when SCAP is complicated with MODS, and oxygen, non-invasive mechanical ventilation or invasive mechanical ventilation should be given to correct respiratory failure if combined with respiratory failure. Those who are malnourished should be given active nutritional support; those who are immunocompromised should be given immune support, such as Ritalin, propecia globulin, etc. 4.Prevention of SCAP: Influenza vaccination and Streptococcus pneumoniae multivalent vaccination can help reduce the incidence of CAP in vulnerable groups, such as the elderly, children, patients with chronic diseases and certain special occupations. Influenza viruses can directly invade the lower respiratory tract, disrupting defenses and leading to CAP or SCAP, and influenza vaccination can reduce the incidence of influenza and thus CAP or SCAP. Studies have shown that Streptococcus pneumoniae polyvalent vaccine significantly reduces the prevalence of Streptococcus pneumoniae bacteremia and pneumonia.