Pneumonia. X-ray of the chest of a patient with bacterial pneumonia (grainy white area, centre right) in the lower lobe of the left lung (black). The heart is at lower centre to lower right (white). Bacterial pneumonia, which results in inflammation of the lung, 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. It is most common after a cold, especially in those with a weakened immune system. Symptoms include fever and chills, a cough with rusty-coloured or pus- like sputum and chest pain. Treatment is with antibiotics.