Chances of Andromeda Strain?

[J.S. Hebert]

The Andromeda Strain was some sort of self-replicating eukaryote or 
prokaryote that was for a short time living off something in the blood, 
and then mutated into something that was no longer leathal or harmful.

What are the chances that humans will come into contact with something 
like that?  Not likely...

The lethality of the two virus you mention actually prevent them from 
spreading, why? because they kill the host before they really become 
infections.  However in today's world where people can easily move from 
the jungle to major metropolises in one day the chances are increasing.  
But most likely the next pandemic will come from a new strain of 
influenza that was created in somesort of migrating bird such as ducks 
and gained a grip in the human population through immune deficient 
individuals.  This virus can easily make species jumps and can be spread 
through casual contact by replicating in the respatory system, unlike 
the other pandemic viruses which are quite difficult to catch: HIV1, and 
2.