This is a theory on why Halo 5 Guardians could see the return of The Flood. This comes to us courtesy of YouTuber HaloFollower, whom you can subscribe to here.
First off is based on Halo Escalation issue 6. There is a panel that shows the cryobay of Spirit of Fire, with one lone Flood infection form, surrounded by open pods. This may be a repeat of the situation in the original Halo Combat Evolved.