According to some surveys, the most common pathogens are pneumococcal types 1, 2, 3, 4, 6, 7, 8, 12, 14, 18 and 19, while in children, types 1, 6, 14, 18, 19 and 23 are more common. Carriers are the main source of infection, and patients play a small role in the source of infection. The bacteria are discharged through nasopharyngeal secretions and spread in the form of droplets. There are occasional outbreaks in dense crowds, which are generally sporadic. The disease occurs in all seasons, but is more common in winter and spring. Males are more likely to get sick than females. Secondary pneumonia is more common in children, the elderly and chronic patients. The normal human upper respiratory tract has a defense barrier, so it is not easy to get infected. Once the barrier is damaged, the defense function is reduced and the disease may occur. For example, upper respiratory tract viral infection can increase mucus secretion, inhibit the function of ciliated epithelium and the role of phagocytes, anesthesia can weaken the epiglottal reflex and lead to aspiration of gastric contents and nasopharyngeal secretions containing bacteria, heart failure, septic shock, and chest trauma can cause water accumulation in the alveoli, bronchial cancer, bronchiectasis, and lung abscess can cause bronchial obstruction and poor drainage, all of which can induce this disease.