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. (“How the Lungs Work” article.) Sometimes, though, germs manage to enter the lungs and cause infections. This is more likely to occur when:

 

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 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. 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 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 World is the bacterium Streptococcus pneumoniae, or pneumococcus.

 

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:

 

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 in Southern California and the desert Southwest, histoplasmosis in the Ohio and Mississippi River Valleys, and cryptococcus3. 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, formerly Pneumocystis carinii, is sometimes considered a fungal pneumonia. However, it’s not treated with the usual antifungal medicines. It usually affects people who:

 

Other kinds of fungal infections also can lead to pneumonia.

 

"Strength does not come from physical capacity. It comes from an indomitable will."

 

Who Is At Risk for Pneumonia?

Pneumonia can affect people of all ages. However, two age groups are at greater risk of developing pneumonia:

 

Other conditions and factors also raise your risk for pneumonia. You’re more likely to get pneumonia if you have a lung disease or other serious disease. Examples include cystic fibrosis, asthma, COPD (chronic obstructive pulmonary disease), bronchiectasis, diabetes, heart failure, and sickle cell anemia.

 

You’re at greater risk for pneumonia if you’re in a hospital intensive-care unit, especially if you’re on a ventilator (a machine to help you breathe).

 

Having a weak or suppressed immune system also can raise your risk. A weak immune system may be the result of a disease such as HIV/AIDS. A suppressed immune system may be due to an organ or bone marrow transplant, chemotherapy, or long-term steroid use.

 

Your risk also goes up if you have trouble coughing because of a stroke, trouble swallowing, limited ability to move, alcohol use, or sedation (being given medicine to make you relaxed or sleepy).

 

Smoking cigarettes, abusing alcohol, and being undernourished also raise your risk for pneumonia.

 

Your risk also goes up if you’ve recently had a cold or the flu, or if you’re exposed to certain chemicals, pollutants, or toxic fumes.