Pneumonia is an infection and inflammation of the lungs.
Pneumonia is the fifth leading cause of death, especially in the very young and the elderly. Atypical pneumonia is usually a flu-like illness caused by Mycoplasma pneumoniae. Legionnaire's disease may also cause atypical pneumonia. In addition, this term is used to describe cases of pneumonia caused by viruses, fungi, or parasites. Bacterial pneumonia is most commonly caused by Streptococcus pneumoniae.