One of the most satisfying aspects of They Are Billions is that its maps are populated by a finite amount of the infected. Clearing areas really does clear them away; they aren’t arbitrarily spawned in, other than as a result of the ten hordes you’ll be facing. From Doom Villages will emerge certain blends of infected that depend on their sizes. Smaller ones will yield weak Decrepit infected, while larger will house fast Runners. In the largest villages, you’ll face special infected. Harpies can jump over your walls, for instance, while the Chubby is incredibly resilient. The ten hordes you face will often also gather wandering infected and bring them along for the ride, so it’s always a good idea to thin out numbers as much as possible during quieter times.