Pneumonia is inflammation (swelling) of the tissue in one or both of the lungs. It’s usually caused by an infection – most commonly, bacteria and viruses, which are contagious. It can also be caused by breathing in a foreign object (aspiration pneumonia) or, in rare cases, a fungal infection, which is not usually contagious.
Causes of pneumonia
Bacterial pneumonia
The most common cause of pneumonia in adults is a bacterium called Streptococcus pneumoniae. Other types of bacteria that can cause pneumonia include:
- Haemophilus influenzae
- Staphylococcus aureus
- Mycoplasma pneumoniae
Read more information about other
bacteria that can cause pneumococcal infections.
Viral pneumonia
Viruses can also cause pneumonia – most commonly, the respiratory syncytial virus (RSV), and sometimes the flu (influenza) type A or B virus.
Fungal pneumonia
In the UK, pneumonia caused by a fungal infection of the lungs is rare in people who are normally healthy. Pneumocystis jirovecii pneumonia remains the most common fungal pneumonia in people with low immune systems, such as those infected with HIV.
Aspiration pneumonia
Pneumonia can also be caused by breathing in a foreign object, such as a piece of food, or a harmful substance, such as smoke. This is called aspiration pneumonia. If breathing in a foreign object leads to a bacterial infection, this is contagious.
Catching pneumonia
The germs that can cause pneumonia are usually breathed in. People often have small amounts of germs in their nose and throat that can be passed on through:
- coughs and sneezes – these launch tiny droplets of fluid containing germs into the air, which someone else can breathe in
- touching an object and transferring germs onto it – someone else can touch this object, and then touch their own mouth or nose
Streptococcus pneumoniae bacteria, which causes most cases of pneumonia, is far less contagious than a
cold or
flu. This is because most people’s immune systems can kill the bacteria before they have the opportunity to cause an infection.
In rare cases, pneumonia can develop from an infection somewhere else in the body, when the germs enter the lungs through the bloodstream.
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