What is the cause of pneumonia?
The micro-organisms that cause pneumonia may be present in your body for some time before you become ill. Alternatively you may breathe in droplets that are in the air from other people coughing and sneezing. You may also breathe in organisms from the environment (for example in Legionnaire’s disease, a form of pneumonia caused by Legionella pneumophilia). The most common bacterium that causes community-acquired pneumonia is Streptococcus pneumoniae – this is called pneumococcal pneumonia. Bacteria that commonly cause hospital-acquired pneumonia (also called nosocomial pneumonia) include Haemophilus influenzae, Klebsiella pneumoniae and Staphylococcus aureus. Viruses, including the influenza (flu) virus can also cause pneumonia. Pneumonia may also be caused by inhaling substances, for example, vomit, a peanut, a chemical or smoke. It can also occur from allergy, for example. ‘Farmers lung’ is from an allergy to mouldy hay. If children, particularly those under school age, get pneumonia
