Causes

Bacteria, viruses, and fungi infections can cause pneumonia. These infections cause inflammation in the air sacs, or alveoli, of the lungs. This inflammation causes the air sacs to fill with fluid and pus.

Bacteria

Bacteria are the most common cause of pneumonia in adults. Many types of bacteria can cause bacterial pneumonia. Streptococcus pneumoniae or pneumococcus bacteria are the most common cause of bacterial pneumonia in the United States.

If your pneumonia is caused by one of the following types of bacteria, it is called atypical pneumonia.

• Legionella pneumophila. This type of pneumonia sometimes is called Legionnaire's disease, and it has caused serious outbreaks. Outbreaks have been linked to exposure to cooling towers, whirlpool spas, and decorative fountains.
• Mycoplasma pneumoniae. This is a common type of pneumonia that usually affects people younger than 40 years old. People who live or work in crowded places like schools, homeless shelters, and prisons are at higher risk for this type of pneumonia. It's usually mild and responds well to treatment with antibiotics. However, Mycoplasma pneumoniae can be very serious. It may be associated with a skin rash and hemolysis. This type of bacteria is a common cause of “walking pneumonia”.
• Chlamydia pneumoniae. This type of pneumonia can occur all year and often is mild. The infection is most common in people 65 to 79 years old.

Bacterial pneumonia can occur on its own or develop after you've had a viral cold or the flu. Bacterial pneumonia often affects just one lobe, or area, of a lung. When this happens, the condition is called lobar pneumonia.

Lobar bacterial pneumonia. This figure shows pneumonia affecting the single lower lobe of the left lung. Figure A shows the location of the lungs and airways in the body. Figure B shows normal alveoli. Figure C shows infected alveoli or air sacs.

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Most of the time, the body filters bacteria 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 help stop the germs from reaching your lungs. Read How the Lungs Work for more information.

Sometimes bacteria manage to enter the lungs and cause infections. This is more likely to occur if:

• Your immune system is weak
• A germ is very strong
• Your body fails to filter out the bacteria from the air that you breathe. For example, if you can't cough because you've had a stroke or are sedated, bacteria may remain in your airways.

When bacteria reach your lungs, your immune system goes into action. It sends many kinds of cells to attack the bacteria. These cells cause inflammation in alveoli (air sacs) and can cause these spaces to fill up with fluid and pus. This causes the symptoms of pneumonia.

Viruses

Viruses that infect the respiratory tract may cause pneumonia. The influenza or flu virus is the most common cause of viral pneumonia in adults. Respiratory syncytial virus (RSV) is the most common cause of viral pneumonia in children younger than one year old.  Other viruses can cause pneumonia such as the common cold virus known as rhinovirus, human parainfluenza virus (HPIV), and human metapneumovirus (HMPV).

Most cases of viral pneumonia are mild. They get better in about one to three 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.

Fungi

Pneumocystis pneumonia is a serious fungal infection caused by Pneumocystis jirovecii. It occurs in people who have weak immune systems due to HIV/AIDS or the long-term use of medicines that suppress their immune systems, such as those used to treat cancer or as part of organ or blood and marrow stem cell transplant procedures.

Other fungal infections also can lead to pneumonia. The following are three fungi that occur in the soil in some parts of the United States and can cause some people to get pneumonia.

• Coccidioidomycosis. This fungus is found in Southern California and the desert Southwest. It is the cause of valley fever.
• Histoplasmosis. This fungus is found in the Ohio and Mississippi River Valleys.
• Cryptococcus. This fungus is found throughout the United States in bird droppings and soil contaminated with bird droppings.