Doctor unwise to inject fluid with infection (5,3)
I believe the answer is:
swine flu
'infection' is the definition.
(swine influenza is an infectious disease)
'doctor unwise to inject fluid' is the wordplay.
'doctor' indicates anagramming the letters.
'to inject' is an insertion indicator (inserted letters are injected).
'fluid' becomes 'fl' (eg in fl oz, fluid ounce).
'unwise' is an anagram of 'swineu'.
'swineu' placed around 'fl' is 'SWINE FLU'.
'with' is the link.
(Other definitions for swine flu that I've seen before include "Bug" , "hazard!" .)