Viruses, bacteria, or (in rare
cases) parasites or other organisms can cause
- 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 pneumoniae is sometimes mild and called
- Viruses, such as
influenza A (the flu virus) and
respiratory syncytial virus (RSV) can cause
In people with
impaired immune systems, pneumonia may be caused by
other organisms, including some forms of fungi, such as Pneumocystis jiroveci (formally called Pneumocystis carinii). This fungus frequently causes
pneumonia in people who have AIDS. Some doctors may
HIV test if they think that Pneumocystis jiroveci is causing the pneumonia.
How do you get pneumonia?
You may get pneumonia:
- After you breathe infected air particles into your
- 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
- As a complication of a viral illness, such as
- 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
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
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.
E. Gregory Thompson, MD - Internal Medicine
R. Steven Tharratt, MD, MPVM, FACP, FCCP - Pulmonology, Critical Care Medicine, Medical Toxicology
March 17, 2011
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