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Pneumonia - Cause

Viruses, bacteria, a fungus, or (in rare cases) parasites or other organisms can cause pneumonia.

  • In most cases, the specific organism (such as bacteria or virus) cannot be identified even with testing.1 When an organism is identified, it is usually the bacteria Streptococcus pneumoniae.2
  • Many types of bacteria may cause pneumonia. Pneumonia caused by Mycoplasma pneumoniaeMycoplasma pneumoniae is sometimes mild and called "walking pneumonia."
  • Viruses, such as influenza A (the flu virus) and respiratory syncytial virus (RSV) can cause pneumonia.

In people who have impaired immune systems, pneumonia may be caused by other organisms, including some forms of fungi, such as Pneumocystis jiroveci (formerly called Pneumocystis carinii). This fungus frequently causes pneumonia in people who have AIDS. Some doctors may suggest an HIV test if they think that Pneumocystis jiroveci is causing the pneumonia.

Recommended Related to Lung Disease/Respiratory Problems

LAM Lung Disease (Lymphangioleiomyomatosis)

LAM lung disease (lymphangioleiomyomatosis) is a rare lung disease that tends to affect women of childbearing age. In LAM lung disease, muscle cells that line the lungs' airways and blood vessels begin to multiply abnormally. These muscle cells spread into areas of the lung where they don't belong. The air sacs in the lung also swell and form small pockets called cysts. As the cysts develop throughout the lungs, LAM causes breathing problems similar to emphysema. The muscle cells can spread...

Read the LAM Lung Disease (Lymphangioleiomyomatosis) article > >

How do you get pneumonia?

You may get pneumonia:

  • After you breathe infected air particles into your lungs.
  • After you breathe certain bacteria from your nose and throat into your lungs. This generally occurs during sleep.
  • During or after a viral upper respiratory infection, such as a cold or influenza (flu).
  • As a complication of a viral illness, such as measles or chickenpox.
  • If you breathe large amounts of food, gastric juices from the stomach, or vomit into the lungs (aspiration pneumonia). This can happen when you have had a medical condition that affects your ability to swallow, such as a seizure or a stroke.

A healthy person's nose and throat often contain bacteria or viruses that cause pneumonia. Pneumonia can develop when these organisms spread to your lungs while your lungs are more likely to be infected. Examples of times when this can happen are during or soon after a cold or if you have a long-term (chronic) illness, such as chronic obstructive pulmonary disease (COPD).

You can get pneumonia in your daily life, such as at school or work (community-associated pneumonia) or when you are in a hospital or nursing home (healthcare-associated pneumonia). Treatment may differ in healthcare-associated pneumonia, because bacteria causing the infection in hospitals may be different from those causing it in the community. This topic focuses on community-associated pneumonia.

    This information is produced and provided by the National Cancer Institute (NCI). The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via the Internet web site at http:// cancer .gov or call 1-800-4-CANCER.

    WebMD Medical Reference from Healthwise

    Last Updated: September 09, 2014
    This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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