Sorry if this has been mentioned but i basically quick scanned the majority of this topic, but flood are NOT playable, confirmed by Graeme himself several times, theres an air unit to counter what the covenant and UNSC have, there in skirmish as being ''there'' like on the map, ambient life as someone said, but they can attack you and grow threw your men.

 Now what im about to say is completely from my thoughts but i would assume they grow through your bases and vehicles and if your careless with your buildings and units, i would assume this can turn into a major disadvantage with them growing very powerful and a major threat to all players.