Festering Wound is a uncommon Enchantment — Aura card from Urza's Destiny (UDS), collector number 61. Current market price: $0.29 (foil $4.50), TCGplayer market prices via Scryfall.
Enchant creature At the beginning of your upkeep, you may put an infection counter on this Aura. At the beginning of the upkeep of enchanted creature's controller, this Aura deals X damage to that player, where X is the number of infection counters on this Aura.