Pneumonia is most commonly caused by an infection, usually a bacterial infection.
However, many different bacteria, viruses and (rarely) fungi cause pneumonia; the germ depends on where the pneumonia began. For example, germs that cause pneumonia caught in hospitals are different to those that cause pneumonia caught in the community.
The germs that cause an infection are usually breathed in. In rare cases, pneumonia can develop from an infection elsewhere in your body, when germs enter your lungs through your bloodstream.
The four types of pneumonia are described in more detail below.
The most common cause of pneumonia in adults is a bacterium called Streptococcus pneumoniae. This form of pneumonia is sometimes called pneumococcal pneumonia.
Read more about pneumococcal infections.
Less commonly, other types of bacteria can cause pneumonia, including:
- Haemophilus influenzae
- Staphylococcus aureus
- Mycoplasma pneumoniae (outbreaks tend to occur every four to seven years, more commonly in children and young people)
And rarely, the following bacteria can cause pneumonia:
- Chlamydophila psittaci: this causes a rare form of pneumonia called psittacosis, which can be passed on to people from infected birds such as parrots, parakeets, pigeons, canaries and budgies (this condition is also called parrot fever or parrot disease)
- Chlamydophila pneumoniae
- Legionella pneumophila: this causes Legionnaires' disease, an uncommon form of pneumonia
Viruses can also cause pneumonia, most commonly the respiratory syncytial virus (RSV), and sometimes the flu (influenza) type A or B virus.
Viruses are a common cause of pneumonia in young children.
Rarely, pneumonia can be caused by breathing in:
- a foreign object, such as a peanut
- a harmful substance, such as smoke or a chemical
The object or substance inhaled causes irritation in the lungs or damages them. This is called aspiration pneumonia.
In the UK, pneumonia caused by fungal infection of the lungs is rare in people who are normally healthy. It more often affects people whose immune systems are weakened (see below). While still rare, fungal pneumonia is more likely to affect people who travel to places where these infections are more commonly found, such as parts of the US, Mexico, South America and Africa.
The medical names for fungal pneumonia include histoplasmosis, coccidioidomycosis and blastomycosis.
People at risk
The following people have a higher risk of developing pneumonia:
- babies and very young children
- elderly people
- people who smoke
- people with other health conditions
- people with a weakened immune system
Health conditions that increase the risk of pneumonia developing include:
- another lung condition such as asthma or cystic fibrosis
- a heart condition
- a kidney or liver condition
- a lowered immune system
Your immune system can be weakened because of:
- a recent illness such as flu
- treatment for cancer, such as chemotherapy
- medicines that weaken the immune system after an organ transplant
HIV or AIDS