Cheapest priced copy (Mirrodin Besieged): $1.29 at Card Kingdom, near mint, their price list as of .
Septic Rats 
Creature — Phyrexian Rat · 2/2 · Uncommon · Mirrodin Besieged · #55
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.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
1 printing · $1.29 to $4.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mirrodin Besieged | 55 | Uncommon | $1.29 | $4.99Out of stock |
2 rulings from Wizards of the Coast
- 1 June 2011A player is poisoned if that player has one or more poison counters.
- 1 June 2011Septic Rats gets a maximum of +1/+1 from its ability, no matter how many poison counters the defending player has.