The term infiltrate is used to describe the processes which cause an increase in lung density. The term infiltrate is a generic one. The description of the abnormality is important as it allows us to determine a differential diagnosis of the abnormality. Interstitial infiltrates have the appearance of line (reticular opactities) or nodules. Alveolar infiltrates have the appearance of hazy coalescent opacities with ill-defined borderd. They tend to obliterate the soft-tissue-air interface of the mediastinal and diaphragmatic borders. Most bacterial penumonias will have the appearance of an alveolar infiltrate. Viral pneumonias tend to have the appearance of an interstitial infiltrate. However, there are many non-infectious causes for both abnormalities.