Many germs can cause pneumonia. Examples include different kinds of bacteria, viruses, and, less often, fungi.
Most of the time, the body filters germs out of the air that we breathe to protect the lungs from infection. Your immune system, the shape of your nose and throat, your ability to cough, and fine, hair-like structures called cilia (SIL-e-ah) help stop the germs from reaching your lungs. (For more information, go to the Health Topics How the Lungs Work article.)
Sometimes, though, germs manage to enter the lungs and cause infections. This is more likely to occur if:
For example, if you can't cough because you've had a stroke or are sedated, germs may remain in your airways. ("Sedated" means you're given medicine to make you sleepy.)
When germs reach your lungs, your immune system goes into action. It sends many kinds of cells to attack the germs. These cells cause the alveoli (air sacs) to become red and inflamed and to fill up with fluid and pus. This causes the symptoms of pneumonia.
Bacteria are the most common cause of pneumonia in adults. Some people, especially the elderly and those who are disabled, may get bacterial pneumonia after having the flu or even a common cold.
Many types of bacteria can cause pneumonia. Bacterial pneumonia can occur on its own or develop after you've had a cold or the flu. This type of pneumonia often affects one lobe, or area, of a lung. When this happens, the condition is called lobar pneumonia.
The most common cause of pneumonia in the United States is the bacterium Streptococcus (strep-to-KOK-us) pneumoniae, or pneumococcus (nu-mo-KOK-us).
Another type of bacterial pneumonia is called atypical pneumonia. Atypical pneumonia includes:
Respiratory viruses cause up to one-third of the pneumonia cases in the United States each year. These viruses are the most common cause of pneumonia in children younger than 5 years old.
Most cases of viral pneumonia are mild. They get better in about 1 to 3 weeks without treatment. Some cases are more serious and may require treatment in a hospital.
If you have viral pneumonia, you run the risk of getting bacterial pneumonia as well.
The flu virus is the most common cause of viral pneumonia in adults. Other viruses that cause pneumonia include respiratory syncytial virus, rhinovirus, herpes simplex virus, severe acute respiratory syndrome (SARS), and more.
Three types of fungi in the soil in some parts of the United States can cause pneumonia. These fungi are:
Most people exposed to these fungi don't get sick, but some do and require treatment.
Serious fungal infections are most common in people who have weak immune systems due to the long-term use of medicines to suppress their immune systems or having HIV/AIDS.
Pneumocystis jiroveci (nu-mo-SIS-tis ye-RO-VECH-e), formerly Pneumocystis carinii, sometimes is considered a fungal pneumonia. However, it's not treated with the usual antifungal medicines. This type of infection is most common in people who:
Other kinds of fungal infections also can lead to pneumonia.
Clinical trials are research studies that explore whether a medical strategy, treatment, or device is safe and effective for humans. To find clinical trials that are currently underway for Pneumonia, visit www.clinicaltrials.gov.
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The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.