what causes pneumonia
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).acteria and viruses are the primary causes of pneumonia. When a person breathes pneumonia-causing germs into his lungs and his body's immune system cannot otherwise prevent entry, the organisms settle in small air sacs called alveoli and continue multiplying.