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.
The problem with consigning God to this role is that you quickly have to leave the pages of the bible to support this view, and at that point why bother to explain it anyway since you have just discredited your source?
Miracles as found in the bible aren't merely unlikely natural processes, they are supernatural. Some have the appearance of nature, but when you study the ten plagues of Egypt you find that natural appearance is just a direct challenge to the gods of the Egyptions. For example, the river turning to blood shows the power of God over that of the god of the Nile.