Players cannot play as members of the Horde, although when playing as one of the lesser special infected (i.e. When they manage to get close, you may need to bash them away, throw a pipe bomb or boomer bile to distract them, or swing a melee weapon at them. If they charge in a single file line, they're easily eliminated with rapid fire weapons, but can also be torched as they run through a flaming area.
When they arrive in groups of 20 infected or more, you will have to defend yourself quickly. These zombies are generally the first responders to any alarm, whether from a car, building, or from any other source of loud noise. Fortunately, their head is very weak, and one shot from any weapon to the head will kill them. Although they are weak individually, they often make up for it in sheer number. However, they are most certainly not slow - they can outrun the survivors. The Horde are by far the most common enemy, and are little more than zombified versions of ordinary people.