Wow, from reading the story and reading every post so far I've learned quite a lot from this.

I remember reading in the story that the owner of the shop had a Red Tail on his shoulders when he went to go get Jade, so he didn't wash his hands or even put the snake back when he went to get Jade so the person can handle her.

That might've spread something to Jade, but something also could have been in her from being in a shop with a reputation of selling diseased snakes.