Septic Rats (MBS) from Mirrodin Besieged, available at 401 Games Canada.

• Rarity: Uncommon
• Finish: Normal
• Artist: Cos Koniotis
• Card Text:
  Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
  Whenever Septic Rats attacks, if defending player is poisoned, it gets +1/+1 until end of turn.