Acute lobar pneumonia. Coloured chest X-ray showing acute lobar pneumonia in the upper lobe of a patient's lung (yellow area in top left). Lobar pneumonia is caused by certain strains of the bacterium Streptococcus pneumoniae, and usually affects a single lobe of one of the lungs. Alveoli (air sacs) in the lobe become blocked with pus, preventing air flow and causing the affected tissue to solidify. Symptoms include fever, painful breathing and a cough that produces rust- coloured sputum. In most cases antibiotic drugs help the patient to make a full recovery, but the disease can be fatal in the elderly or debilitated.