That Days Gone true ending man...NERO was injecting their own personnel with the virus, plus, with Deacon injecting himself with the "experimental" drugs throughout the game (NERO boosters)...There were some clear implications that by the 2nd game he'd also be infected and slowly but surely turning into a "super mutant" himself, that, or it could have been the game's "twist" at the end.
Maybe they could have had some game mechanics revolving around that in the form of abilities like super-jumps/tougher skin/souped up Witcher vision (enhanced smelling)/faster movement etc etc...
Either way, it's fascinating stuff since there's lots that could have been done with the game's lore.
What a friggin shame, this, IMO, was Sony's best open world game surpassing even games like Ghost of Tsushima.
Unfortunately , we're gonna get fed with GOW, Marvel shit and GaaS till the end of times from what it seems, sad turn of events really.