Pneumonia is a serious infection involving one or both of the lungs. When the lungs become infected, the air sacs (alveoli) fill with pus, mucus, and other liquids and then cannot function properly. When this happens, oxygen cannot reach the blood and if there is not enough oxygen in the blood, body cells cannot function properly and may even die.  

Lobar pneumonia affects a section (lobe) of a lung, and bronchial pneumonia (or bronchopneumonia) affects patches throughout both lungs.