Sure it can. With the sort of lead times you're thinking of, it'll be spread globally. All you have to do to see who did it is to look around and see who's not sick. (They would have to innoculate themselves before deploying something like that. And BTW, that sort of massive innoculation program would itself be pretty difficult to hide or disguise.)
Something that risky, they might accept some collateral damage for the end result. Everyone and their cuzzin leroy has gotten into bios, my guess is, like every other major weapon ever invented, they'll be used someday extensively. Biology is spooky now, that goat that gives industrial fiber spider web silk in it's milk is beyond amazing. Who knows what's shaking in the premier labs, I certainly don't, other than probably a lot more than what we really want to know about...