Snape Vampire Theory: Where did this come from?

[Kelly]

No: HPFGUIDX 77553

I have been reading the Snape Vampire theories for a long time and I, 
personally, do not subscribe to them.  Most of the responses on the 
matter harp on how the cannon supports, or detracts from, this 
theory. However, I think it would be interesting to discuss the 
following:

First, assume that Snape IS, in fact, a vampire (no matter how you 
feel).  

1.  How is this relevant to the plot and how does this aid to further 
the development of the books?  

2.  Does this directly affect the course of the books or the final 
outcome of book 7?

I am actually very interested to hear the theories on this. It must 
be the latent English teacher in me. 

Cheers!
Kelly