Pneumonia. Coloured frontal X-ray of the chest of a young girl with bacterial pneumonia (grainy white areas) in the lungs. The heart is at lower centre (white). Bacterial pneumonia, which results in inflammation of the lung, is most commonly caused by the bacteria Streptococcus pneumoniae, but this variant is caused by Staphylococcus bacteria. 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.