Septic Rats is a uncommon Creature — Phyrexian Rat card from Mirrodin Besieged (MBS), collector number 55. Current market price: $0.61 (foil $3.53), TCGplayer market prices via Scryfall.
Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.) Whenever this creature attacks, if defending player is poisoned, it gets +1/+1 until end of turn.