A 22 year old college student comes in with gradual onset headache, malaise, sore throat, low-grade fever, and chills. He also has an intractable nonproductive cough. He says other people around him have had a similar illness. Physical exam is unremarkable. You present the case to the attending, sure it is a benign upper respiratory tract infection.
Challenge: Your attending who has a particular interest in infectious disease says: if this patient tested positive for cold agglutinins, what organism would you suspect?