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What Causes Pneumonia?

Pneumonia can be caused by a wide variety of bacteria, viruses and fungi in the air we breathe. Identifying the cause of your pneumonia can be an important step in getting the proper treatment.

Bacteria

The most common type of bacterial pneumonia is called pneumococcal pneumonia. Pneumococcal pneumonia is caused by the Streptococcus pneumoniae germ that normally lives in the upper respiratory tract. It infects over 900,000 Americans every year.

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 part, or lobe, of a lung. When this happens, the condition is called lobar pneumonia. Those at greatest risk for bacterial pneumonia include people recovering from surgery, people with respiratory disease or viral infection and people who have weakened immune systems.

Some types of bacteria cause what is known as "atypical" pneumonia, including:

  • Mycoplasma pneumoniae, a tiny wide-spread bacterium that usually infects people younger than 40 years old, especially those living and working in crowded conditions. The illness is often mild enough to go undetected and is sometimes referred to as walking pneumonia.
  • Chlamydophila pneumoniae, which commonly causes upper respiratory infections year-round, but can also result in a mild form of pneumonia.
  • Legionella pneumophila, which causes a dangerous form of pneumonia called Legionnaire's disease. Unlike other bacterial pneumonias, Legionella is not passed from person to person. Outbreaks of the disease have been linked to exposure to contaminated water from cooling towers, whirlpool spas, and outdoor fountains.

These bacteria are referred to as "atypical" because pneumonia caused by these organisms might have slightly different symptoms, appear different on a chest X-ray, or respond to different antibiotics than the typical bacteria that cause pneumonia. Even though these infections are called "atypical," they are not uncommon.

Viruses

Viruses that infect the upper respiratory tract may also cause pneumonia.  The influenza virus is the most common cause of viral pneumonia in adults. Respiratory syncytial virus (RSV)  is the most common cause of viral pneumonia in young children. Most viral pneumonias are not serious and last a shorter time than bacterial pneumonia.

Viral pneumonia caused by the influenza virus may be severe and sometimes fatal. The virus invades the lungs and multiplies; however, there are almost no physical signs of lung tissue becoming filled with fluid. This pneumonia is most serious in people who have pre-existing heart or lung disease and pregnant women. Read about the connection between the flu and pneumonia.

Viral pneumonias may be complicated by a secondary invasion of bacteria, with all the typical symptoms of bacterial pneumonia.

Fungi

Fungal pneumonia is most common in people with chronic health problems or weakened immune systems, and in people who are exposed to large doses of certain fungi from contaminated soil or bird droppings.

Pneumocystis pneumoniais 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 manage organ transplants.

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.

    Reviewed and approved by the American Lung Association Scientific and Medical Editorial Review Panel. Last reviewed December 31, 2018.

    Page Last Updated: February 14, 2019

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