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