Causes of pneumonia

Many different kinds of bacteria and viruses can cause pneumonia.

If pneumonia is caused by bacteria or viruses, it’s contagious – you can catch it from another person.

The most common cause of pneumonia is a bacterium called streptococcus pneumoniae. It’s much less contagious than flu or a cold, because most people’s immune systems can kill it before it causes an infection.

More people get pneumonia in winter. This is because other infections that spread easily from person to person, such as flu, are more common in the winter. Catching flu can increase your risk of developing pneumonia.

Next: Symptoms of pneumonia >


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Last medically reviewed: October 2016. Due for review: October 2019

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.