Pneumonia is inflammation of the lung that is most often caused by infection with bacteria, viruses, or other organisms. Occasionally, inhaled chemicals that irritate the lungs can cause pneumonia. Healthy people can usually fight off pneumonia infections. However, people who are sick, including those who are recovering from the flu (influenza) or an upper respiratory illness, have a weakened immune system. This makes it easier for bacteria to grow in their lungs. It is one of the top 10 causes of death in the U.S. and the 6th leading cause of death in the elderly (65 and older).
Defining Pneumonia by Location in the Lung
Pneumonia may be defined according to its location in the lung:
- Lobar pneumonia occurs in one part, or lobe, of the lung.
- Bronchopneumonia tends to be scattered throughout the lung.
Defining Pneumonia by Origin of Infection
Doctors often classify pneumonia based on where the disease is contracted. This helps predict which organisms are most likely responsible for the illness and, therefore, which treatment is most likely to be effective.
Community-Acquired Pneumonia (CAP). People with this type of pneumonia contracted the infection outside a hospital setting. It is one of the most common infectious diseases. It often follows a viral respiratory infection, such as the flu.
Review Date: 04/13/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.