I've read in a wiki that the flood cant infect hunters or the yenme because the hunsters exist out of small worms which lack the body structure to be infected and yenme cant be infected because they have an exo skeleton and in order to be infected by the flood you need normal bones which contain lots of calcium so the flood can form those into tentacles and stuff.
So those wont be in the game but grunts and jackals however can be infected and i surely hope they will put these in