Pneumonia is an infection that affects the air sacs in the lungs, causing inflammation. Pneumonia can cause the air sacs to fill with fluid or with pus.
Pneumonia can be caused by bacteria, viruses, fungi, or by inhaling a liquid or certain chemicals. Pneumonia can be quite mild, but it can also be very serious or even life threatening. People who have the highest risk of serious or life-threatening pneumonia are infants and young children, people over the age of 65, and people with weakened immune systems.