Also, a parasite can spread quite fast depending on how it is deployed. It'll most likely be used as a bioweapon (since nature has way better ways of killing humans) ensuring a wide starting basis. If it's blood based , there's a large chance it can be spread around by biting insects. If the paraste also finds itself into your various body fluids(saliva) it might also spread by pets and pests and such. Combined with a long incubation period this means that the parasites can break most quarantines, and can remain pasively present in the environements for many years.
"Also, virusses that are that deadly have been found (and created) and have been Quarantined without problems."
I don't doubt it, but has there ever been an outbreak of a bio-weapon? Also, examples of the former. Please, please let's scratch bloody zombies. Let's think about how it was for the first, and first small group of, homo sapiens, which must have been utterly fascinating.
The British once tested an Anthrax bioweapon on a small island. 40 years later, they firebombed it, dug the soil off and replaced it with soil from the bottom of the sea. It's still not noted on many maps. This anthrax bombardement meant that any mammal that would walk on the island without proper equipment would die within a few days.
This was during the second world war btw, and scientist estimated that if they bombed german cities this way it would make them uninhabitable for decades.
http://en.wikipedia.org/wiki/Gruinard_Island.
There have, as far as I know, been no official acknowledgements of any bioweapon outbreaks.
Edit: Not firebombed. Sprayed with metanal.