Board Thread:The Reference Desk/@comment-188432-20130505035914/@comment-188432-20130506174530

SmallerOnTheOutside wrote: Could you perhaps list the instances in which you think the script's referring to the leech or the venom as the Crimson Horror?
 * VASTRA: … and our greatest plague, the most virulent enemy, was the repulsive red leech
 * DOCTOR: Oooooh.  "The repulsive red leech"!  Nah, on balance, I think I prefer "the Crimson Horror". What was it, exactly?
 * VASTRA: A tiny parasite. It infected our drinking water.  And once in our systems, it secreted a fatal poison.
 * DOCTOR: So it's been hanging around, lurking in the shadows. Maybe it's evolved.  Maybe it's had help.

To me, this pretty clearly shows that they're talking about the creature being the Crimson Horror. I draw from this scene that the red leech is the Crimson Horror, but in other cases it's more the manner of death — as Doug points out – "ginned up" by the penny-dreadfuls.