Pneumonia - Causes

What are the causes of pneumonia?

Pneumonia is commonly caused by an infection with a germ. The germ may be a bacterium or a virus. There are three or four different bacteria that are the most common causes of pneumonia. There is also a well-known group of bacteria that causes pneumonia in about 3 out of 10 cases. They are called atypicals. Other germs such as fungi, yeasts, or protozoa can sometimes also cause pneumonia.

Rarely, non-infective pneumonia is caused by inhaling poisons or chemicals. Many different substances can cause this. They can be in the form of liquids, gases, small particles, dust or fumes. 

You may breathe in some bacteria, viruses, or other germs. If you are normally healthy, a small number of germs usually doesn't matter. They will be trapped in your sputum and killed by your immune system. Sometimes the germs multiply and cause lung infections. This is more likely to happen if you are already in poor health - for example:

  • If you are frail or elderly.
  • If you have a chest disease.
  • If you have a low immunity to infection. Low immunity can be caused by such things as alcohol dependence, AIDS, or another serious illness.

However, even healthy people sometimes develop pneumonia.

A particular type of pneumonia is known as aspiration pneumonia. Small amounts of stomach contents or liquid produced in the mouth or throat can be inhaled into the lungs. The inhaled substance can be very irritating to the lungs, cause infection or block the smaller airways. Aspiration pneumonia usually happens in frail, elderly people, people who are drowsy or unconscious, or people who have conditions which cause swallowing difficulties.

How do you diagnose pneumonia?

  • Symptoms - a doctor will suspect pneumonia from asking about your symptoms and how you are feeling. They may also ask about your medical history and that of your family. They will be interested in whether you smoke, how much and for how long. The examination may include checking your temperature. Sometimes your doctor will check how much oxygen is circulating around your body. This is done with a small device that sits on the end of your finger. The doctor will listen to your chest, so they may want you to lift or take off your top. If you want a chaperone during the examination, the doctor will arrange one. If you have asthma, they may ask you to check your peak flow measurement. They will listen to your chest with a stethoscope. Tapping your chest over the infected lung is also sometimes performed. This is called percussion. An area of infected lung may sound dull. 
  • X-ray - a chest X-ray may be required to confirm the diagnosis and to see how serious the infection is.
  • Other tests - these tests are usually carried out if you need to be admitted to hospital. They include sending a sample of phlegm (sputum) for analysis and blood cultures to check if the infection has spread to your blood.

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Author:
Dr Laurence Knott
Peer Reviewer:
Prof Cathy Jackson
Document ID:
4491 (v42)
Last Checked:
20 January 2015
Next Review:
19 January 2018

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.