Small pox

[Steve Quan]

Ok,ok I'm just your average layman  with an interest in teeny tiny 
bugs.  I feel stupid asking, but here goes.  Can anyone tell me WHY 
small pox is no longer a threat?  I understand about the eradication 
campaign and that the last case was reported sometime in the late 70's. 
 Isn't the bug itself still out there somewhere (besides the stuff in 
deep freeze)? How do they know it's gone?  If answers would be too long 
for this post can anyone recommend a book?  Pardon my ignorance and 
thanks.