I'm assuming a contagious agent. Stuff such as anthrax is limited to the spores released. It isn't replicated in the host and passed to additional victim. The hemorrhagic fever viruses such as ebola and Marburg spread quickly and kill quickly. In a 3rd world country with limited transportation systems, the fever blows out the entire population of a tiny geographic area. The "epidemic" is self limiting because of sparse population and limited contact. Move that same virus to a dense city such as New York, San Francisco or Chicago and you have a favorable set of conditions to wipe out a few million people.
Ah, but you are assuming this is true. Bush has been trying to drum up support for his invasion of Iraq for quite some time. He wants to finish the job his old man left undone, and then we'll be stuck in the middle of hell for decades to come. We can't even establish peace and reasonable order in Afghanistan. How many more countries are we going to invade and "rebuild"?