Cheapest priced copy (Urza's Destiny): $0.39 at Card Kingdom, near mint, their price list as of .
Festering Wound 
Enchantment — Aura · Uncommon · Urza's Destiny · #61
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.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.39 to $4.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Urza's Destiny | 61 | Uncommon | $0.39 | $4.49 |
2 rulings from Wizards of the Coast
- 4 October 2004If it is on your creature, you can decide whether to add a counter before or after it damages you.
- 4 October 2004Putting on a counter is optional. If you forget, you can't go back later even if it is something you usually do.