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.
A player is poisoned if that player has one or more poison counters.
Septic Rats gets a maximum of +1/+1 from its ability, no matter how many poison counters the defending player has.