To get the The Purge achievement in State of Decay 2 do the following:
Infestations are shown on your map and mini-map as green, infected looking houses or other structures. When you approach them you'll see HUD information telling you how many regular zombies and screamers are present.
Prioritize the screamers as soon as you can, or else they'll just keep attracting more zombies (plus their screams immobilize you if you're too close when they start shrieking). Like hordes, infestations are quite common so this shouldn't take too long. When you've cleared out ten of them, this achievement will unlock.