So the Greek for wound is [h]elkos, and the lexicon says it's typically used of a wound that exudes puss. I don't have one of the larger Greek Lexica anymore in which I could confirm that.
sore and grevious
kakon + poneron
Literally just means bad and onerous, but it could refer to loathesome, disgusting, troublesome, etc. when combined with wound. I'd need the much larger Lexicon to be sure.
But I think that festering sores might not be too far off.
Why does it afflict just those who received the mark? Was it supernaturally directed or somehow a natural consequence of the "Mark"?