At Patient, we know our readers sometimes want to have a deep dive into certain topics. In this series of articles centred around pneumonia, you can read about the causes, symptoms and treatments - all written by one of our GP experts.

What causes pneumonia?

Pneumonia is almost always caused by bugs - viruses, bacteria, fungi - getting into the lungs and causing an infection. Normally, the immune system stops infections entering into the lungs and living there. However, if infections do successfully attack the lungs, the immune system's response causes the affected areas to fill with fluid or pus, leading to the symptoms of pneumonia.


Bacterial infections - the most common causes of pneumonia - are sometimes divided into 'typical' and 'atypical' infections. Typical bacteria are the commoner ones that cause pneumonia and atypical are the less common ones.

Typical bacteria that cause pneumonia include:

Atypical bacteria that cause pneumonia include:

  • Legionella pneumophila - the cause of Legionnaires' disease. Legionella bacteria can contaminate water sources, such as air conditioning units and hot tubs, and may cause pneumonia if inhaled.
  • Mycoplasma pneumoniae. This commonly can cause pneumonia in people under 40 but can affect all age groups. It is also known as walking pneumonia, because it can cause milder symptoms than other types of pneumonia.
  • Chlamydia psittaci - a bacterium that infects birds, and which can be spread to humans who are in close contact with birds - especially parrots.


Viral pneumonia is also common. Causes include:

Sometimes, a viral infection starts first, and then a bacterial pneumonia develops - called secondary bacterial pneumonia. An example of this is bacterial pneumonia - often caused by Staphylococcus aureus - occurring after having the flu (influenza infection).


In the UK, fungal pneumonia usually affects people with weakened immune systems, such as those taking immune-suppressing medicines or people with AIDS.

Causes of fungal pneumonia include:

  • Pneumocystis jirovecii - which causes pneumocystis pneumonia (PCP). PCP is closely linked with AIDS, and also occurs in people who have severe immune system problems for another reason, such as after bone marrow transplants. Treatments available for HIV are now very effective in treating the virus and preventing AIDS, so PCP is less common than it used to be.
  • Aspergillus moulds - moulds found in soil, plants and trees, dust, air conditioner units, and damp buildings.

See Fungal Lung Infections for more information.


Pneumonia due to parasites is very rare in the UK, but sometimes occurs in people with weakened immune systems. In some tropical countries, parasitic pneumonia are more common - for example, Leishmaniasis from sand flies.

Non-infectious causes

There are some rare conditions which cause inflammation and scarring in the lungs, but are not caused by an infection. Examples are cryptogenic organising pneumonia and non-specific interstitial pneumonia. In most cases, the cause of these is unknown. These conditions can lead to lung fibrosis.


This is a rare form of pneumonia, but If harmful chemicals are inhaled or otherwise get into the lungs, they can cause irritation and damage. For example, breathing in high levels of chlorine gas can cause lung damage - see Aspiration pneumonia.

How common is pneumonia?

Each year, around one in 100 adults in the UK will develop pneumonia. Pneumonia is most common in older people - affecting around two in 100 people who are aged 80 or over. It also affects younger children - around one in every 200 under five year olds each year.

Types of pneumonia

Pneumonia can be classified in different ways - such as by the type of infection causing it (including bacteria, viruses and fungi - see causes above).

Pneumonia can also be divided into different types, depending on how the infection was caught. This can help decide how to treat the pneumonia.

Types of pneumonia include:

Community-acquired pneumonia

This is the most common type of pneumonia, and describes pneumonia that is caught outside of hospital.

Hospital-acquired pneumonia

People who are admitted to hospital may develop pneumonia whilst there. Hospital-acquired pneumonia is usually defined as a pneumonia that occurs five or more days after the patient has been admitted to hospital. There are often more dangerous organisms in hospitals - such as MRSA - and people who have been admitted are usually more unwell and less able to fight off infections. Hospital-acquired pneumonia can therefore be more serious, and requires different antibiotics to treat it.

Ventilator-associated pneumonia

This is a type of hospital-acquired pneumonia that can affect people who are on ventilators - breathing machines - who are usually already very unwell and in intensive care. Ventilator-associated pneumonia can be very serious as a result.

Aspiration pneumonia

This occurs when food or liquid from the mouth or stomach enters the lungs, bringing in acid and bacteria. Usually, swallowing reflexes and coughing prevent aspiration pneumonia from happening. When these are absent, or not working as well, harmful bacterial from the mouth or stomach can enter the lungs.

Common reasons for this to happen are:

How can pneumonia spread?

Most bacteria and viruses that cause pneumonia are spread from person to person by coughing, sneezing, or talking near someone else. People nearby can breathe in droplets that contain the bacterium or virus that causes pneumonia, and sometimes these can cause pneumonia in other people - the likelihood of this depends on the exact organism. Viruses tend to be more contagious than bacteria.

In some cases, people carry bacteria or viruses in their throat, which can spread to the lungs - possibly by swallowing tiny amounts of them.

Some types of pneumonia, such as most fungal pneumonias, are spread by inhaling fungal spores in the air. People with weakened immune systems are most likely to be affected.

Some types of pneumonia are spread through specific ways: for example, Legionella bacteria live in water sources, such as air conditioning units, and can be inhaled and cause infection in the lungs.

Risk factors for pneumonia

Pneumonia can affect anyone. However, some people are at higher risk of developing pneumonia, such as:

How to prevent pneumonia

The best way to prevent pneumonia is by vaccination. Several vaccines are available for children and adults and provide good protection against pneumonia, including protecting against serious illness and death. There are vaccines for:

Stopping smoking and reducing alcohol intake can help prevent against pneumonia - and also have lots of other benefits.

For people with HIV, antiviral treatments are very effective in treating the virus and preventing the immune system from becoming weakened and vulnerable to pneumonia.

Some people with severe immune system problems might be advised to take regular antibiotics to prevent pneumonia and other infections. For example, young children with sickle cell disease are given regular penicillin to prevent pneumococcal infection.

Washing your hands regularly, especially after coughing, sneezing and before eating or preparing foods can help prevent the spread of infections including pneumonia.

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