What Causes Pneumonia? | American Lung Association

What Causes Pneumonia?

Many different germs and other things can cause pneumonia. There are five main causes of pneumonia:

  • Bacteria
  • Viruses
  • Mycoplasma
  • Other infectious agents, such as fungi—including pneumocystis
  • Various chemicals

If you have viral pneumonia, you also are at risk of getting bacterial pneumonia.

Types of Pneumonia

Bacterial Pneumonia

Bacterial pneumonia can attack anyone, at any age. It can occur on its own or develop after you've had a cold or the flu. People at greatest risk for bacterial pneumonia include people recovering from surgery, people with respiratory diseases or viral infections and people who have weakened immune systems.
If your body's defenses are weakened—by illness, old age, malnutrition, or impaired immunity—the pneumonia bacteria, which can live in healthy throats, can multiply and work their way into the lungs. The infection can quickly spread through the bloodstream and invade the entire body.
Dozens of different types of bacteria can cause pneumonia:

  • The most common type of bacterial pneumonia is pneumococcal pneumonia, affecting over 900,000 Americans every year. Children under the age of 5 and adults over 65 have a recommendation to receive a pneumococcal vaccination series to help protect them against the Streptococcus (pneumococcus) bacteria.
  • Atypical pneumonia is caused by bacteria such as Legionella pneumophila, Mycoplasma pneumoniae, and Chlamydophila pneumoniae.
  • Other bacteria that can cause pneumonia include Staphylococcus aureus, Moraxella catarrhalis, Streptococcus pyogenes, Neisseria meningitidis, Klebsiella pneumoniae, and Haemophilus influenzae.

Viral Pneumonia

Most respiratory viruses attack the upper respiratory tract, but some cause pneumonia, especially in children.  Most of these pneumonias are not serious and last a short time but others can be severe.

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.

In extreme cases, the patient has a desperate need for air and extreme breathlessness. Viral pneumonias may be complicated by an invasion of bacteria, with all the typical symptoms of bacterial pneumonia.

Mycoplasma Pneumonia

Mycoplasma are the smallest free-living agents of disease in humankind. They are not classified as to whether they are bacteria or viruses, but they have traits of both.

Mycoplasma usually cause a mild form of pneumonia, but may be severe with certain symptoms lasting a long time. They affect all age groups, but occur most often in older children and young adults.

Other Types of Pneumonia

Tuberculosis (TB) rarely can cause pneumonia (tuberculosis pneumonia). It is a very serious lung infection and extremely dangerous unless treated early.

Pneumocystis jiroveci pneumonia, which is caused by a fungus, is sometimes seen in people whose immune system is impaired (due to HIV infection or certain medications that suppress the immune system).

Other less common pneumonias may be quite serious and occur more often. Various special pneumonias are caused by the inhalation of food, liquid, gases or dust.

Learn more about diagnosing and treating pneumonia »


    Approved by Scientific and Medical Editorial Review Panel. Last reviewed October 6, 2016.

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