At the beginning of your upkeep, you may put an infection counter on Festering Wound.
At the beginning of the upkeep of enchanted creature's controller, Festering Wound deals X damage to that player, where X is the number of infection counters on Festering Wound.
10/4/2004 If it is on your creature, you can decide whether to add a counter before or after it damages you.
10/4/2004 Putting on a counter is optional. If you forget, you can’t go back later even if it is something you usually do.