Pneumonia is an inflammation of the airspaces (alveoli; singular alveolus) within the lung most ordinarily caused by infections. Bacteria, viruses, or fungi (infrequently) can cause the infection. There also are a couple of noninfectious sorts of pneumonia that are caused by inhaling or aspirating foreign matter or toxic substances into the lungs.

Some cases of pneumonia are life-threatening. Around 50,000 people die annually of pneumonia within the U.S. Although anyone of any age are often affected, pneumonia is more common in elderly people and sometimes occurs when the system becomes weakened via a previous infection or another condition.

Pneumonia is usually more serious when it affects older adults, infants and young children, those with chronic medical conditions, or those with weakened immune function.