Pneumonia is caused by inflammatory lesions in the lung parenchyma due to infection, physical and chemical stimulation, and immune damage. 1. Lung infection: Pathogens invade lung tissue and cause infectious lesions, which is the most common cause of pneumonia. Pathogens that can cause pneumonia include bacteria, viruses, mycoplasma, chlamydia, fungi and so on. 2. Physical and chemical stimulation: some foreign objects are inhaled into the lungs, such as dentures, date kernels, etc., the physical and chemical stimulation caused to the lungs, is also the cause of pneumonia. 3. Immune damage: the immune system within the body is damaged, is a cause of pneumonia, this immune damage may be organ transplantation, tumor chemotherapy, leukemia, lymphoma, AIDS and other causes. Nowadays, pneumonia is categorized into many different types such as hospital-acquired pneumonia, community-acquired pneumonia, pneumonia in the elderly, pneumonia in children, and so on, according to the place of origin of the disease as well as the status of the host. Individuals who are affected by pneumonia should seek medical attention as soon as possible.