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?.