Among our population at HUP, pneumocystis pneumonia is the most common cause of an interstitial pneumonia. To my eye, pneumocystis pneumonia most commonly appears as ground glass opacity. This is a term I have not used previously but it implies a faint excess density which does not obscure the normal interstitial markings. The term viral pneumonia is quite broad covering a wide variety of organisms. Most viral pneumonias in the adult have little or no radiographic findings.
Children are more likely to have increased interstitial markings in viral pneumonias.
Chlamydia trachomatis is the cause of a congenitally acquired pneumonia in newborn infants which is usually manifest after a few weeks of life. This organism is transferred to the infant as he or she passes through the birth canal. This characteristically results in an interstitial pneumonia. Chlamydia is also responsible for Q fever in the adult but this is usually alveolar in appearance. These are listed in the table.