A single festercreep isn't alone. It's already an infestation.

Festercreep

Festercreep comes into play with a +1/+1 counter on it.
{1B}, Remove a +1/+1 counter from Festercreep: All other creatures get -1/-1 until end of turn.

This creature enters with a +1/+1 counter on it.
{1}{B}, Remove a +1/+1 counter from this creature: All other creatures get -1/-1 until end of turn.

The List

Commander Anthology Volume II

Festercreep enters the battlefield with a +1/+1 counter on it.
{1}{B}, Remove a +1/+1 counter from Festercreep: All other creatures get -1/-1 until end of turn.