Most pneumonia occurs when a breakdown in your body's natural defenses allows germs to invade and multiply within your lungs. To destroy the attacking organisms, white blood cells rapidly accumulate. Along with bacteria and fungi, they fill the air sacs within your lungs (alveoli). Breathing may be labored. A classic sign of bacterial pneumonia is a cough that produces thick, blood-tinged or yellowish-greenish sputum with pus.