[@LegionpothIX]

True, but think of this, if and when the galaxy erupts into chaos, the number of host species will increase. I've played enough halo to get the sense of scale and intelligence the flood posses. So, I can somewhat attest to how strong they are, they are so intelligent that they turned a Contender class A.I. (An A.I. that is smart enough to almost be called alive) against the Forerunners (The ones who killed the Precursors, the gods of their universe). In fact, you could start by infecting another species other than the Humans or at a place where their influence is so weak that you would just be considered just a plague.

I can actually see multiple options for you to snowball, but I can also see the information - cold war style approach working out for you. Getting people to fight each other just to assimilate their biomasses later when they are too weak to fight you off, or take their dead.