Strange file -- a nun catching unknown disease (9)
I believe the answer is:
influenza
'disease' is the definition.
(influenza is a kind of disease)
'strange file a nun catching unknown' is the wordplay.
'strange' indicates anagramming the letters.
'catching unknown' becomes 'za' (I can't justify this - if you can you should give a lot more credence to this answer).
'file'+'nun'='filenun'
'filenun' anagrammed gives 'influen'.
'influen'+'za'='INFLUENZA'
Can you help me to learn more?
(Other definitions for influenza that I've seen before include "Illness once called grippe" , "Highly infectious viral disease" , "Winter virus disease, even birds get it" , "Viral infection" , "Common viral disease" .)