Pneumonia. Coloured X-ray of the chest of a patient with bacterial pneumonia (pink, centre right) in the lower lobe of the left lung (dark blue). The heart is at centre right (orange). 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.