A little bit detailed:
What is it?
The air sacs in the lungs are filled with pus and other liquid which causes a lack of oxygen in the body. Because of this (and eventually spreading infection) may cause death. Until 1936, pneumonia was regarded the No.1 cause of death: in 1998, the infectious disease is ranked as sixth in the leading causes of death
Pneumonia bacteria are present in every 'healthy person', but can 'attack'
if the body has been weakened by serious illness, old age, wrong food on a daily
basis or alcohol. Streptococcus pneumoniae is the most common cause of bacterial pneumonia.
Probably half of the pneumonia variants are caused by viri, esp. in children.
Most of these variants are shortlasting, though.
Mycoplasmas are the smallest living agents of disease unclassified being viri or bacteria.
Generally they cause light forms of pneumonia.
Other less known variants can be caused by fungi, gases, food or dust. Foreign 'bodies' in your
body (like tumors) may be 'promoting' pneumonia too.