Phyresis is a common Enchantment — Aura card from Mirrodin Besieged (MBS), collector number 49. Current market price: $2.32 (foil $18.80), TCGplayer market prices via Scryfall.
Enchant creature Enchanted creature has infect. (It deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)