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What causes pneumonia?

Pneumonia is caused by an infection of the lung. Most infections are due either to bacteria or viruses, although often a cause is never found.

The bacteria that most commonly cause pneumonia are known as Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. The most common viruses are the influenza virus, human adenovirus and the respiratory syncytial virus. It is thought about half of pneumonia cases are caused by a virus.

Another common cause of pneumonia is infection by mycoplasma, a kind of bacterium. Pneumonia caused by mycoplasma organisms is usually milder but it can take longer to recover from. Other organisms, such as fungi, can also cause pneumonia. This is more common in people whose immune systems aren’t working properly, such as those with HIV infection or those being treated for cancer.

Some people are more likely to get pneumonia or develop a more severe illness, including:

  • babies and young children
  • people over 65
  • people with lung diseases, such as asthma, cystic fibrosis or chronic obstructive pulmonary disease (COPD)
  • people with weak immune systems or who are taking medications that suppress their immune systems
  • people with heart, kidney or liver disease
  • Aboriginal and Torres Strait Islander people
  • people who smoke
  • people with feeding problems, such as those with intellectual disabilities at risk of aspiration (drawing food or liquid into the airway)

If you suspect that you or someone in your care may have pneumonia, it is important to see a doctor as soon as possible. If the affected person has trouble breathing, they should go to the nearest hospital emergency department.

Last reviewed: February 2017

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