Subject: Where did smallpox come from?

Where did smallpox come from?
I have read recently that smallpox was the first disease that was 
treated out of existense.  I understand that there are only two 
samples of smallpox still in existense, one in some laboratory in 
Russia and one in the United States.  I was always under the 
impression that the only way smallpox could be contracted was
from someone who already had it.  If this is true, how did the
first person who ever had smallpox get it?  And, if that happened
once, whu can't it happen again?

Re:Where did smallpox come from?