What Is Pneumonia? Pneumonia is a serious lung infection that causes the air sacs in the lungs to fill with fluid or pus.As a result, breathing becomes difficult and the oxygen level in the blood can drop. It can be caused by bacteria, viruses, or fungi. However, children under five years old are most vulnerable because their immune systems are still developing.Therefore, recognizing pneumonia symptoms early and getting timely medical tests can help prevent complications. Common Pneumonia Sympto...