With your consent, Edmund pricks your butt and draws some blood. It's going fine until...
"Uh-oh. Um, I don't know how to tell you this, but I may have... oh, how to put it... well..."
You feel a finger rub your ass, and as it does you hear the squeak of latex. That's not a good sign.
"Yes, well, you see, it seems that the lowered amount of blood back here after I'd taken my sample was just enough to prevent your body from properly fighting the infection. Oh, but don't worry! I'm sure it'll stop as soon as it progresses beyond this area here... I think..."
Mortified, you grab your ass cheeks and feel as their human flesh slowly becomes that smooth rubbery latex. It consumes your entire ass and even a bit of the area around it, and then you feel a slight bulge near the top of the latex area that slowly expands until it's taken the unmistakable form of a canine tail.
"There, see? That wasn't that bad, was it?" Not that bad?!? Did he miss the fact that your ass is now smooth and purple, did the TAIL that now hangs from you escape him somehow? "Oh my, the tail's quite rigid right now. I take that to mean you're angry. Well, come now, look at the bright side: at least the changes have stopped progressing."