Anthrax (of which Iraq already has the supply, and the delivery system - as of '98, FAS reported that Iraq had 8,400 liters of this, and 19,000 liters of), botulinim, brucella (developed as a weapon by the U.S.), ebola, rotavirus, smallpox, bubonic & pneumonic plague...take your pick.
If you want to be especially nasty, you can even make a cocktail, say, ebola's effects grafted onto the high-contagion capability of smallpox.