When counterterrorism officials compare notes on what keeps them awake at night, the weapon they most fear is not a nuclear bomb or poison gas, but a disease that was wiped off the planet 20 years ago. They worry about smallpox, a lethal, super-contagious virus that, if unleashed, would spread quickly through an unprotected world population.

The Bush administration is exploring the possibility of expanding the nascent U.S. missile-defense system into Eastern Europe as a protection against an attack from the Middle East.