Frontal (left) and lateral (right) chest X-ray of a 30 year old female patient showing lobar pneumonia (white) in the medial lobe of her right lung. Pneumonia can be caused by bacteria or viruses. The alveoli (the air sacs of the lungs, too small to be seen) become blocked with pus, forcing air out and causing the lung to solidify. Symptoms include coughing and chest pain.