Could they have co-existed? (Spoilers)
If Melanie had allowed Caldwell to kill her and make a vaccine, humans could have been made immune to the airborne spores and would no longer have been at risk for turning into hungries. But the hungries that already exist would still be around.
Assuming the humans then went on to wipe out all the 1st generation hungries (who can't be reasoned with), could they have tried to make peace with the 2nd generation hungries and co-existed from that point on as two human species sharing the earth? Melanie could be reasoned with; she had enough self-control to make the decision to hunt animals and not humans. Why not the others?
Conceivably, they could have built a society with two kinds of people, one carnivorous and one omnivorous.