Pneumonia is mainly caused by pathogenic microorganisms, such as bacteria, viruses, atypical pathogens, fungi, etc., and can be caused by other non-infectious factors, such as radiation injury, allergic reactions, etc. 1. Bacterial infection: Streptococcus pneumoniae is the most common pathogen of bacterial pneumonia, and other pathogens such as Staphylococcus aureus, Haemophilus influenzae, and Klebsiella pneumoniae can also cause pneumonia. 2. Viral infection: it is the most common cause of pneumonia in children under 5 years old, mainly including respiratory syncytial virus (RSV), parainfluenza virus (type 1, 2, 3), influenza virus (type A, type B) and measles virus. 3. Atypical pathogen infections: mainly including Mycoplasma (Mycoplasma pneumoniae), Chlamydia (Chlamydia trachomatis, Chlamydia pneumoniae), etc. 4. Fungal infections: mainly including Candida albicans, Cryptococcus and Pneumocystis, etc. 5. Non-infectious causes: such as radioactive injury, or food, drink, vomit and other foreign objects are inhaled into the lungs when an allergic reaction occurs, can cause pneumonia. Pneumonia occurs in relation to the above factors, so daily should try to avoid going to densely populated places and close contact with respiratory tract infections, if there is a suspicion of pneumonia symptoms, should also promptly and actively seek medical treatment.