Viruses, bacteria, or (in rare cases) parasites or other organisms 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 pneumonia 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 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 suggest an HIV test if they think that Pneumocystis jiroveci is causing the pneumonia.

Transmission

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 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, such as during or soon after a cold or if you have a long-term (chronic) illness, such as chronic obstructive pulmonary disease.

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.