If the aliens existed and infested just ONE city, that town would be nuked after the initial scare wore off and the government figure out what they are up against.
Of course not all of the Aliens would die and they would move on. Plus the radiation from the nuke would spread out to neighboring cities.
Some of the people who were face hugged would leave town too, and run away to another city for "safety". Another outbreak would spread there.
Same thing with Zombie bites. Someone is going to run away or their families will try to get them to some doctor to heal them despite their better judgment. Aliens will eventually force humanity into a few heavily isolated and protected cities.
