Pneumonia is an inflammation of one or both lungs, usually caused by an infection. It causes the alveoli (air sacs) inside the lungs to fill with fluid, making it harder for them to work properly.
The body sends white blood cells to fight the infection, and while this helps kill the germs it can also make it harder for the lungs to pass oxygen into the bloodstream.
Some people with mild pneumonia can manage the condition at home, while those with more severe infections may require hospital treatment. People who develop pneumonia during a hospitalisation for a different medical problem have a high risk of becoming very ill.