That only affects the firstborn? I mean, if you are going to throw out the main details of the story, why not throw out the whole thing?
I always assumed that God used natural agents for the plagues. Beyond that he would have "managed" the circumstances as needed to achieve His end, such as who got infected.
It's just how I picture God working. For example, if God feels the need for a big meteor to strike Sodom on a given date, I have no problem with him causing the chain of events that leads to that a few million years ago.
But then you get into that whole predestination thing and have to ask about the Sodomites free will. But with a name like "Sodomite" you really gotta know nothing goods gonna happen (thats meant as a joke).
Lovely little movie about this idea with Tom Conti and Teri Garr, callled Miracles, of all things.