An Infestation is when a fairly large group of zombies take up residence in a specific location. These occur randomly, and you can find these in 1 of 2 ways.
- If you've already "found" the building by scouting, it will appear as an Infestation on your map once it becomes Infested.
- By going near the Infested location, this will add it to your map. If you do not deal with Infestations, you will be given the mission, "Seek and Destroy", which is a good indicator that there are several Infestations nearby.
Infestations usually have a few Screamers, though more can spawn, and several zombies. The best way to lure zombies out is to use either a cop car or firecrackers/timer/etc, then running the zombies over as they come running.
If you're having trouble finding Infestations, try looking in whatever town you're NOT in. I was near the Courthouse and was having trouble finding new Infestations, but when I went back to the Church, I found 3, and when I went back to the Ranger Station (where you start), I found 6.
I recently started Breakdown, and you'll be able to get quite a few Infestations quickly. If you're willing to start, restart, and so on, you can unlock it this way too.