Pneumonia. Illustration of the interior of a lung showing alveoli (air sacs), in which gaseous exchange takes place, that are inflamed and filled with pus (orange). Pneumonia is an infection of the lungs, most commonly caused by the bacteria Streptococcus pneumoniae, although it can be caused by other bacteria, viruses, or fungi. The pathogen is inhaled into a lung segment, or lobe, where it causes the alveoli to become blocked with pus. Symptoms include fever and chills, a cough with rusty-coloured or pus-like sputum and chest pain.