Board Thread:The Reference Desk/@comment-188432-20130505035914/@comment-188432-20130507182202

Fair enough. (Though, if I can make the obvious joke, Anoted, I don't think this episode was set in Alabama.)  If I can take a stab at stating the apparent consensus of the thread:
 * Crimson Horror - the disease, as named by the penny-dreadfuls. Capped because that's how the papers at the time would almost certainly have named it, for extra sensation.
 * The Crimson Horror - change the redirect to The Crimson Horror (TV story)
 * red leech - the parasite
 * red leech poison (both the Doctor and Vastra independently call it "poison", though the Doctor says, "poison, a kind of venom", so therefore a redirect at red leech venom is probably valid.

I would however, like a BTS note at red leech and Crimson Horror to the effect that it is possible to red the scene denotatively to suggest that the Crimson Horror could be the leech. I think a tophat at Crimson Horror to red leech is also reasonable.

Objections?