Virulent disease leads to extremely resilient bug (6)
I believe the answer is:
pester
'bug' is the definition.
(synonyms)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for pester that I've seen before include "Trouble persistently" , "Nag, badger" , "Importune" , "Bug" , "Harass" .)