Community-acquired pneumonia (CAP) is an inflammation of the infected lung parenchyma (including the alveolar wall, i.e., the interstitial lung in the broad sense) that develops outside the hospital, including pneumonia that develops within an average incubation period after hospital admission due to pathogenic infection with a defined incubation period. In today’s antibiotic era, CAP remains an important disease threatening human health, especially due to the aging of the society’s population, the increase in immune-compromised hosts, pathogenic variability and rising rates of antibiotic resistance, which pose many challenges to CAP diagnosis and treatment. There are approximately 3-5.6 million cases of CAP each year in the United States, with a direct medical cost of $8.4-9.7 billion [1]. In the United Kingdom, CAP is the most common cause of emergency admissions and is the leading cause of death from infectious disease. However, the management of pneumonia by clinicians remains highly irregular, with many confusing antimicrobial treatments and random drug use, which not only increases the economic burden, but also causes increased selective pressure for antibiotics and increased resistance rates. Therefore, countries have developed and continuously updated CAP guidelines to standardize the diagnostic and therapeutic process, reduce mortality, and decrease health care costs. Diagnosis of CAP In patients with a recent onset of cough, sputum and/or dyspnea, especially with fever, altered breath sounds, or the presence of CAP, older patients or immunocompromised patients often do not have fever, but only present with confusion, depression, or exacerbation of pre-existing underlying disease, but these patients often have increased respiration and abnormal chest physical examination. Patients with suspected CAP can be diagnosed by chest radiographs, which can help observe the presence of lung abscesses, tuberculosis, airway obstruction, or pleural effusion, and can also help evaluate the severity of the disease by looking at the extent of lung lobe involvement. Therefore, all authoritative international guidelines agree that chest radiographs should be performed if a patient is considered for CAP. A proportion of immunocompromised CAP patients have a negative chest radiograph, which is rarely present in immunocompetent adults, although the history and physical examination are highly suggestive, as in approximately 30% of patients with Pneumocystis jiroveci pneumonia. A significant proportion of patients with negative chest radiographs and high clinical suspicion of CAP who underwent high-resolution CT could indeed be found to have infiltrative lung changes [2]. Because the clinical significance of the results of this study is unclear and CT is expensive, most societies, including the Infectious Diseases Society of America (IDSA), do not recommend the routine use of CT [3]. The guidelines for the diagnosis and treatment of community-acquired pneumonia (draft) developed by the Chinese Society of Respiratory Medicine in 1998 state that the clinical diagnosis of CAP is based on (i) a new cough, cough, or aggravation of existing respiratory disease with purulent sputum; with or without chest pain; (ii) fever; and (iii) solid lung signs and/or wet stalls with WBC >10´109/L.