Pneumonia is inflammation of the air sacs (alveoli) of the lungs, usually due to infection. Often, only part of one lung is affected, but in severe cases both lungs can be affected, which may be life-threatening.
Most cases are caused by infection with bacteria, including Streptococcus pneumoniae (also called pneumococcus), causing pneumococcal pneumonia; Legionella pneumophila, causing a form of pneumonia known as Legionnaires’ disease; Haemophilus influenzae; and Mycoplasma pneumoniae.
Viral causes of pneumonia include influenza and chicken pox. In some cases, pneumonia can be caused by fungi or protozoa. These infections may be serious in those with reduced immunity; for example, the protozoan Pneumocystis jirovecii may cause a severe type of pneumonia (Pneumocystis pneumonia) in people with AIDS. Pneumonia may also be caused by inhaling vomit, a form known as aspiration pneumonia.
Symptoms typically include fever, sharp chest pain, shortness of breath, and a cough that produces sputum, which may sometimes be bloody.
Occasionally, the infection may spread from the lungs to the pleura (membranes around the lungs), causing pleurisy, or to the blood, causing blood poisoning (septicemia).
Treatment depends on the cause and severity, but may include medication and, in severe cases, oxygen therapy.
Vaccination against influenza and pneumococcus can help to prevent pneumonia.

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