Primary pneumonia is caused by the patient's inhaling or aspirating a pathogen such as bacteria or a virus. Bacterial pneumonia, often caused by staphylococcus, streptococcus, or klebsiella, usually occurs when the lungs' defense mechanisms are impaired by such factors as suppressed cough reflex, decreased cilia action, decreased activity of phagocytic cells, and the accumulation of secretions. Viral pneumonia occurs when a virus attacks bronchiolar epithelial cells and causes interstitial inflammation and desquamation, which eventually spread to the alveoli.

Secondary pneumonia ensues from lung damage that was caused by the spread of bacteria from an infection elsewhere in the body or by a noxious chemical. Aspiration pneumonia is caused by the patient's inhaling foreign matter such as food or vomitus into the bronchi. Factors associated with aspiration pneumonia include old age, impaired gag reflex, surgical procedures, debilitating disease, and decreased level of consciousness.

Community-acquired pneumonia is caused by bacteria that are divided into two groups: typical and atypical. Organisms that cause typical pneumonia include Streptococcus pneumoniae (pneumococcus) and Haemophilus and Staphylococcus species. Organisms that cause atypical pneumonia include Legionella, Mycoplasma, and Chlamydia species.

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