Tainted Strike is a common Instant card from Scars of Mirrodin (SOM), collector number 80. Current market price: $3.68 (foil $30.51), TCGplayer market prices via Scryfall.
Target creature gets +1/+0 and gains infect until end of turn. (It deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)