Pneumonia. Coloured chest X-ray of a pneumonia infection (black and green) in the lower lobe of a patients right lung (red). Pneumonia, which results in inflammation of the lung, may be bacterial or viral in origin. It is most commonly caused by the bacteria Streptococcus pneumoniae. The pathogen is inhaled into a lung segment, or lobe, where it causes the alveoli (air sacs) to become blocked with pus. Treatment of bacterial pneumonia is with antibiotics, and viral pneumonia with antiviral drugs.