what is pneumonia caused by
Knowledge center. Pneumonia is an infection of the lungs that is caused by bacteria, viruses, fungi, or parasites. It is characterized primarily by inflammation of the alveoli in the lungs or by alveoli that are filled with fluid (alveoli are microscopic sacs in the lungs that absorb oxygen).At times a very serious condition, pneumonia can make a person very sick or even cause death.t is characterized primarily by inflammation of the alveoli in the lungs or by alveoli that are filled with fluid (alveoli are microscopic sacs in the lungs that absorb oxygen). At times a very serious condition, pneumonia can make a person very sick or even cause death.