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Pneumonia and your lungs

Illustration comparing normal lungs and fluid-filled air sacs in people with pneumonia

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, they fill the air spaces within your lungs (alveoli). Breathing may be labored as air spaces become inflamed and filled with fluid. A classic symptom of pneumonia is a cough that produces thick, blood-tinged or yellowish-greenish sputum.

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