It leaves its victims one sting closer to phyresis.

Plague Stinger

Flying
Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)

Phyrexia: All Will Be One Commander

Magic Online Promos

DCI Promos

Scars of Mirrodin

A player who has ten or more poison counters loses the game. This is a state-based action.

Damage from a source with infect affects planeswalkers normally.

Damage from a source with infect is damage in all respects. If the source with infect also has lifelink, damage dealt by that source also causes its controller to gain that much life. Damage from a source with infect can be prevented or redirected. Abilities that trigger on damage being dealt will trigger if a source with infect deals damage, if appropriate.

If damage from a source with infect that would be dealt to a player is prevented, that player doesn't get poison counters. If damage from a source with infect that would be dealt to a creature is prevented, that creature doesn't get -1/-1 counters.

Infect's effect applies to any damage, not just combat damage.

The -1/-1 counters remain on the creature indefinitely. They're not removed if the creature regenerates or the turn ends.