Red Fox will usually have multiple dens. When one gets flea infested they move on to another, often nearby, fifty yards or so away. They use Dens over and over again for generations. Every generation of Fox improves the den some and they can get really big (underground).
I always worry about parasites and Rabies. You can get an oral Rabies inoculation, throw the baits near the Den. The oral inoculations may be free, they are here. Parasites is the big one, Fox carry some really nasty ones. One in particular is incurable if you get it.
Fox will really clean the Ducks, Pheasant and other ground nesting birds out. They deplete the Rabbit population in a hurry.
Like any predator the aim is to not let them overwhelm the local eco system. Fox are good at that, they reproduce until most all of the small game around is gone. Then you'll see starving and sickly Fox getting desperate. A really sad situation.
I typically take twenty a year off my lease and still have no shortage of Fox.
I think of them as large predator Rats, not in a bad way. They are survivors.