Bug is real, it spreads (8)
I believe the answer is:
listeria
'bug' is the definition.
'is real it spreads' is the wordplay.
'spreads' indicates an anagram (I've seen 'spreading of' mean this).
'is'+'real'+'it'='isrealit'
'isrealit' with letters rearranged gives 'LISTERIA'.
(Other definitions for listeria that I've seen before include "Is it real? (anag) -- kind of food poisoning" , "Common pathogenic bacterium" , "food culture" , "Possible source of infection" , "Micro-organisms associated with food poisoning" .)