Pneumonia is a disease of the lungs and the respiratory system. The lung contains many small bulbs, or sacs, called alveoli. These help to take out oxygen from the air. In the case of pneumonia, these bulbs become inflamed. They fill up with a fluid which is called pus, and can no longer absorb as much oxygen as before. This makes it hard for the person with pneumonia to breathe. They may feel out of breath, or like they are drowning. They may also feel pain when they breathe. Sometimes people die of pneumonia, even when they go to a hospital and take medicine.