Whenever a creature dies, if it had a -1/-1 counter on it, put a -1/-1 counter on target creature.
This ability is mandatory. If you’re the only player who controls any creatures when this ability triggers, you must target one of them.
When Blowfly Infestation’s ability resolves, it will put a single -1/-1 counter on the targeted creature. It doesn’t matter how many -1/-1 counters were on the creature that went to a graveyard as long as it had at least one on it.