Pneumonia is swelling (inflammation) of the tissue in one or both lungs. It’s usually caused by a bacterial infection.

At the end of the breathing tubes in your lungs are clusters of tiny air sacs. If you have pneumonia, these tiny sacs become inflamed and fill up with fluid.

The symptoms of pneumonia can develop suddenly over 24 to 48 hours, or they may come on more slowly over several days.

Pneumonia is usually the result of a pneumococcal infection, caused by bacteria called Streptococcus pneumoniae.