Microscopic infectious agent (5)
I believe the answer is:
virus
'microscopic infectious agent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for virus that I've seen before include "Cause of computer malfunctions?" , "agent with bad influence" , "Pathogenic agent" , "Cause of infection (in people and computers)" , "Ultramicroscopic infectious agent" .)