A 25-year-old graduate student presents with fever, chills, and a non-productive cough, having felt unwell for several weeks. Chest x-ray reveals diffuse interstitial markings in both lungs, with negative bacterial sputum cultures and rapid influenza test, but positive cold agglutinins. The most likely cause of his pneumonia is suggested by these findings.

1. A 25 -year-old graduate student enters the student health center with fever, chills, and non-
productive cough. He hasn't been feeling well for a few weeks. A chest X-ray shows diffuse
interstitial markings of both lungs without frank lobar infiltrate. Routine bacterial sputum cultures
are negative, as well as a rapid influenza test. Cold agglutinins are positive. What is the most likely
cause of this pneumonia?