Pneumonia is most commonly caused by an infection, usually bacterial.

However, many different bacteria, viruses and – more rarely – fungi cause pneumonia.

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.

Bacterial pneumonia

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

The following are rare causes of bacterial 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

Viral 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.

Aspiration pneumonia

Fungal pneumonia

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 

Hospital-acquired pneumonia

People in hospital for another condition, or a procedure such as an operation, can develop pneumonia caused by germs acquired from the hospital. This is called hospital-acquired pneumonia.

People on breathing machines in the intensive care unit are at particular risk; this is known as ventilator-associated pneumonia. The pattern of germs is different to other types of pneumonia, and they are sometimes resistant to antibiotics. However, these types of pneumonia can still be treated with antibiotics.