Types of pneumonia

Pneumonia is often divided into two main categories ('community-acquired pneumonia' and 'hospital-acquired pneumonia') depending on whether you were infected while living at home (in the community) or while staying in hospital.

There are different types of pneumonia within these categories caused by various infections, which are described below.

There are a range of bacteria that may cause an infection leading to 'typical pneumonia' including Streptococcus pneumoniae which causes pneumococcal pneumonia. This is the most common cause of pneumonia.

Viruses can also be a source of infection including influenza - or flu.

An infection with a bacterium and a virus can occur at the same time. A Streptococcus pneumoniae infection is usually 'secondary' to a person having flu for example. This is known as a 'secondary infection' and can slow down recovery significantly.

Less commonly, microorganisms can cause other forms of pneumonia. These illnesses are classed as "atypical pneumonia" and include:

Legionnaire's disease (caused by a bacterium)

SARS - severe acute respiratory syndrome (caused by a virus)

People with a weakened immune system (such as people with AIDS or those that have had an organ transplant and are taking immunosuppressant drugs) may get additional pneumonia-causing infections.

These include an infection with the fungus-like organism Pneumocystis carinii. This infection is rarely the cause of pneumonia in people who have a fully-functioning immune system.

Pneumonia can also be caused by inhaling substances, such as caustic chemicals, food or vomit into the lungs. This is known as "aspiration" pneumonia and is not infectious.