A 42 year old male presents with a history of cough and progressive
breathlessness. He has previously been treated with chemotherapy for Hodgkin’s
lymphoma and is in remission. There is no other significant illness, he does
not smoke or drink. He is an accountant married and has 2 children.
On examination you note that he is breathless at rest, pulse 100
beats per minute, JVP not elevated, heart sounds normal, breath sounds
vesicular with fine late inspiratory crepitations audible at both lung bases.
In this patient which one of the following is likely to have caused