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Overview of Pneumonia

by John G. Bartlett, MD

Pneumonia is an infection of the small air sacs of the lungs (alveoli) and the tissues around them.

  • Pneumonia is one of the most common causes of death worldwide.

  • Often, pneumonia is the final illness in people who have other serious, chronic diseases.

  • Some types of pneumonia can be prevented by immunization.

In the United States, about 2 to 3 million people develop pneumonia each year, and 45,000 of them die. Pneumonia is the sixth most common cause of death overall, and the most common fatal infection acquired in hospitals. In developing countries, pneumonia is either the leading cause of death or second only to dehydration from severe diarrhea.

The setting in which pneumonia develops is one of the most important features to doctors. Pneumonia may develop in people living in the community (community-acquired pneumonia), in the hospital (hospital-acquired pneumonia), or in some other institutional setting, such as a nursing home (nursing home–acquired pneumonia). Walking pneumonia is not a type of pneumonia. Rather, it is a term sometimes used to describe community-acquired pneumonia that causes such mild symptoms that the person is able to continue normal activities.

The setting often helps determine what infecting organism is responsible for the pneumonia. For example, community-acquired pneumonia is more likely to stem from infection with the bacterium Streptococcus pneumoniae . Hospital-acquired pneumonia is more likely to be caused by Staphylococcus aureus or a gram-negative bacterium, such as Klebsiella pneumoniae or Pseudomonas aeruginosa . Depending on the infecting organism, there is usually a difference in the severity of pneumonia and the way it is treated (for example, whether with oral drugs at home or with intravenous drugs in the hospital).

Another critical feature is whether the pneumonia occurs in a healthy person or in someone who has an impaired immune system. Certain drugs (such as oral or intravenous corticosteroids) can impair the immune system, as can the presence of diseases, such as AIDS or various types of cancer. Sometimes the immune system can be worn down by a severe acute or chronic illness, as is often the case with older people. A person who has an impaired immune system is far more likely to contract pneumonia, including pneumonia caused by unusual organisms. Also, a person whose immune system is impaired may not respond as well to treatment as someone whose immune system is healthy.

Other conditions that predispose people to pneumonia include alcoholism, cigarette smoking, diabetes, heart failure, and chronic obstructive pulmonary disease. The very young and very old are at higher-than-average risk.

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