What Causes Pneumonia?

Many different germs can cause pneumonia. These 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. (For more information, see the Diseases and Conditions Index “How the Lungs Work” article.) Sometimes, though, germs manage to enter the lungs and cause infections. This is more likely to occur when:

• Your immune system is weak
• A germ is very strong
• Your body fails to filter germs out of the air that you breathe

Your mouth and airways are exposed to germs as you inhale air through your nose and mouth. 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 example, coughing is one way the body keeps germs from reaching the lungs. Some people may not be able to cough because, for example, they’ve had a stroke or are sedated (given medicine to make them sleepy). This means germs may remain in the airways rather than being coughed out.

When germs do 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.

Germs That Can Cause Pneumonia

Bacteria

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.

Dozens of different 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).

Lobar Pneumonia

Figure A shows the location of the lungs and airways in the body. It also shows pneumonia that’s affecting the lower lobe of the left lung. Figure B shows normal alveoli. Figure C shows infected alveoli.

Another type of bacterial pneumonia is called atypical pneumonia. Atypical pneumonia includes:

• Legionella pneumophila. This is sometimes called Legionnaire's disease. This type of pneumonia has caused serious outbreaks. Outbreaks have been linked to exposure to cooling towers, whirlpool spas, and decorative fountains.
• Mycoplasma pneumonia. This is a common type of pneumonia that usually affects people younger than 40. People who live or work in crowded places like schools, homeless shelters, and prisons are most likely to get it. It’s usually mild and responds well to treatment with antibiotics. But, it can be very serious in some people. It may be associated with a skin rash and hemolysis (the breakdown of red blood cells).
• Chlamydophila pneumoniae. This kind of pneumonia can occur all year and is often mild. The infection is most common in people 65 to 79 years of age.

Viruses

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.

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 also.

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.

Fungi

Three types of fungi in the soil in some parts of the United States can cause pneumonia. These fungi are coccidioidomycosis (kok-sid-e-OY-do-mi-KO-sis) in Southern California and the desert Southwest, histoplasmosis (HIS-to-plaz-MO-sis) in the Ohio and Mississippi River Valleys, and cryptococcus (krip-to-KOK-us). 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 as a result of long-term use of medicines to suppress their immune systems or having HIV/AIDS.

Pneumocystis jirovecii (nu-mo-SIS-tis ye-RO-VECH-e), formerly Pneumocystis carinii, is sometimes considered a fungal pneumonia. However, it’s not treated with the usual antifungal medicines. It usually affects people who:

• Have HIV/AIDS or cancer
• Have had an organ and/or bone marrow transplant
• Take medicines that affect their immune systems

Other kinds of fungal infections also can lead to pneumonia.