Parasite essentially libels medic with virus (3,3)
I believe the answer is:
bed bug
'parasite' is the definition.
(bed bug is a type of parasite)
'essentially libels medic with virus' is the wordplay.
'essentially' says to take the centre.
'medic with' becomes 'd' (I can't justify this - if you can you should believe this answer much more).
'virus' becomes 'bug'.
The middle of 'libels' is 'be'.
'be'+'d'+'bug'='BED BUG'
(Other definitions for bed bug that I've seen before include "used in FSB honeytrap?" , "It sucks your blood while you sleep" , "Creepy individual" .)