Lobar pneumonia. Frontal X-ray of a patient's chest showing acute lobar pneumonia. Pneumonia is an inflammation of the lung caused by bacteria (normally Streptococcus pneumoniae). The alveoli, a network of microscopic air sacs in the lung, become filled with pus to the exclusion of air (giving the lung a cloudy appearance). Lobar pneumonia affects a whole lobe or the entire lung. Prompt treatment with antibiotics can cure pneumonia. The patient's heart is seen at lower right.