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What is Pneumonia?

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Pneumonia is a respiratory system disease that affects the lungs. The elderly and young children are the groups most at risk of developing pneumonia. Basic measures to avoid contracting pneumonia include good hand hygiene, quitting smoking and an appropriate vaccination programme.

Pneumonia explained in first person

Professionals and patients explain how you live with the disease
Penumonia | PortalCLÍNIC
Pneumonia can be prevented by good hygiene, such as washing your hands, or using a mask if someone has a productive cough.
Pneumonia explained in first person...
If you’ve been diagnosed with pneumonia, the first thing I would say is to trust your medical team. And the most important thing I recommend, for whatever reason, is not to smoke.

Pneumonia is a lung infection that causes inflammation and damage to lung tissue. The lungs are comprised of passages (bronchi) through which air circulates and small sacs (alveoli) where gas exchange occurs. This gas exchange consists of obtaining oxygen from the air and eliminating carbon dioxide produced within the body.

In a healthy person, the alveoli fill with air during respiration; however in people with pneumonia, the alveoli are already full of pus and fluid which affects gas exchange.

The lungs

The lungs are located in the chest cavity and keep our blood oxygenated through breathing. Upon breathing, air enters the body through the nose or the mouth, goes down the throat and along the larynx and trachea until it reaches the lungs, by means of the main bronchi, and finally the alveoli. Alveoli are small, fine-walled sacs found in close contact with some blood vessels that are responsible for oxygen–carbon dioxide gas exchange (blood is oxygenated and carbon dioxide eliminated).

The heart pumps [deoxygenated] blood from the body to the lungs where it is re-oxygenated in the alveoli. The other side of the heart then pumps this recently oxygenated blood back to the rest of the body. A healthy person breathes between 10 and 15 times per minute.

Types of pneumonia

Pneumonia can be classified into two groups depending on the clinical presentation. This classification refers only to pneumonia acquired outside the hospital.

  • Community-acquired pneumonia (CAP). Community-acquired pneumonia is characterised by a rapid clinical onset, high fever, pain in one side or transient abdominal pain, rapid breathing (tachypnoea), cough and purulent or brownish phlegm. One example of this common type of presentation is pneumonia induced by Streptococcus pneumoniae (pneumococcus), which is considered the main pneumonia-causing microorganism worldwide.
  • Community-acquired pneumonia with an atypical clinical presentation. This is characterised by a less acute onset, low fever, a barely productive (dry) cough, diarrhoea and/or vomiting. Pneumonias caused by intracellular bacteria, e.g., Mycoplasma pneumoniae or Legionella pneumophila, are known as atypical pneumonias due to their unusual presentation.

How many people are affected by pneumonia?

Pneumonia is a very common infection. According to data from the World Health Organization (WHO), in 2015, pneumonia and influenza were collectively the third highest cause of death worldwide.

The incidence of community-acquired pneumonia (CAP) in Europe is estimated to be 2–10 cases per 1,000 inhabitants/year. Children under 5 years and adults over 65 are the most vulnerable groups.

It is calculated that between 20% and 70% of cases in Spain require hospitalisation and approximately 9% of patients diagnosed with pneumonia have to be admitted to an intensive care unit (ICU) for monitoring.

CAP that requires hospital admission results in death in 5–7% of cases, but this can increase to 25% for patients admitted to an ICU.   

Read more Causes and risk factors
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Substantiated information by:

Antoni Torres Martí
Juan Roselló Sáncho
Miquel Ferrer Monreal
Núria Albacar Ingla

Published: 20 February 2018
Updated: 3 July 2025

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