Pneumonia. Coloured frontal X-ray of a patient's chest showing pneumonia of the upper lobe of the right lung (red). Pneumonia is caused by bacterial infection, usually Streptococcus pneumoniae, which results in inflammation of the lung. The air sacs in the lungs (alveoli) become filled with pus to the exclusion of air (giving the lung a cloudy appearance). Prompt treatment with antibiotics can cure pneumonia.