Ever since Halo 3, the Flood have been in a very dormant state. The Spartans of Infinity were trained how to combat the Flood if ever they came back and other Spartans were sent to Installation 03 to neutralize any Flood outbreaks, but no confrontation has been documented. But in Halo Escalation issue 6 we see a lone infection form aboard the Spirit of Fire with the ship on a collision course with an unknown planet. It seems that one way or the other, we will see the Flood again soon. But how does this fit in with Halo 5? There are a few theories I have come up with.
. The Elites are still in a civil war and the UNSC is split up as to whether they should help the Arbiter or coax each side to kill each other. If the planet in the picture of the infection form is Sanghelios, then this could mean a rather detrimental surprise. The Flood could rapidly start to spread amidst the civil war and by the time humanity and the Covenant realize what happening, they have the return of a very serious threat.
. If the unidentified planet is not Sanghelios, then the way the Flood enter the fray will be quite different. The Flood would probably go for the smaller wildlife of the planet and move up the food chain. They would then take the SOF and go to the nearest neighbor planet. The SOF’s crew surviving by escaping on one of the Vessel’s ships or becoming part of the collective would depend on where 343 wants to take the story of the Captain Cutter and the others.
.In this theory the way the Flood returns could go either way. The main point of this theory, though, is the Didact’s possible survival after his confrontation with the Master Chief. If he returns and sees one of the Forerunner’s most deadly enemies spreading across the galaxy once again, he would probably be even more fueled with resentment against humanity and be faced with a few options on what to do with the parasites. He could (1) take the remaining Prometheans he has left, possibly leading the Covenant once again, and attempt to destroy the infestation. (2) Search the galaxy for lost Forerunner weapons like the war sphinx and build a new army, possibly using another weapon akin to the Composer. Or (3) Reluctanly do what the proposition always fought against, use the Rings to wipe out all sentient life in the galaxy. If he believes it would take too long to muster up a force large enough to combat the Flood, this could be a possibility.
Another question would be, If the Flood forms a Gravemind, would we see the same one that was present in Halo 2 and 3 or would it be completely new? The infection form was from the Shield World where the Flood that was from the Forerunner Flood war thrived, the Flood is also a collective. Could this mean that any Gravemind that is formed from the same strain is the same Gravemind of Installation 05?
The Flood are such a fascinating and intimidating enemy and having them in Halo 5 would certainly escalate the danger to an already unstable galaxy.