Breathe Better on X: "Pneumonia is an infection that causes the air sacs (called alveoli) in one or both lungs to fill up with fluid or pus. Find out how it can
Mayo Clinic - Pneumonia is an infection that inflames the air sacs in one or both lungs. The air sacs may fill with fluid or pus (purulent material), causing cough with phlegm
![The New York Times on X: "Human lungs are made up of millions of microscopic, balloon-shaped air sacs called alveoli, the workhorse of the respiratory system, where the exchange of gases takes The New York Times on X: "Human lungs are made up of millions of microscopic, balloon-shaped air sacs called alveoli, the workhorse of the respiratory system, where the exchange of gases takes](https://pbs.twimg.com/media/EXg95QMXgAElcIA.jpg:large)