Infective particle (5)
I believe the answer is:
virus
'infective particle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for virus that I've seen before include "computer problem" , "Infectious particle" , "Cause of computer malfunctions?" , "Organism causing disease, or malicious program in computer" , "Harmful agent to animals and computers" .)