Virus beginning? (4)
I believe the answer is:
germ
This clue is a double definition.
'virus' is the first definition.
(thesaurus)
'beginning?' is the second definition.
(germ is a kind of beginning)
(Other definitions for germ that I've seen before include "Micro-organism causing illness" , "First idea; seed embryo" , "Harmful micro-organism" , "Disease-causing microorganism" , "infectious organism" .)