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Also, in this case, where the special property of the venom is that it keeps wounds open, there is a known specific antidote for it, already in existence. Unless St. Mungo's, you know, destroyed all they made and burned the recipe, then Severus has over two years in which to get himself some one way or another, a year and some months of which he was still a member of the Order, as far as I can tell.