And the Answer Is . . .
...migratory duck. Wild ducks can harbor the Avian Flu virus without getting ill themselves or showing any signs of the diease. They can spread the disease to domestic birds like chickens (and possibly to humans) through their feces and other bodily fluids.
Its been estimated that an Avian Flu pandemic (which could be started by a single wild duck) would cause upwards of 20 million human deaths*.
* Note that "20 million deaths" is just from the Avian Flu, and does not include the millions of additional human deaths caused each year by herds of wild stampeding ducks. ;)