[eno_]

The death rate is about 30%. It would take a minimum of 6 million infections to reach a death rate of "millions." You also assume the epdemic does not spread to parts of the world where it cannot be controlled through quarantines.

[Poohbah]

Please note also that a smallpox outbreak would cause many to voluntarily quarantine themselves.

[Trust but Verify]

I have done some research. The mortality rate varies greatly depending on factors such as strain, vaccination programs, age and overall health of those who contract it.

There is great disagreement on whether those of us who have been vaccinated retain any resistance to the disease.

The part you posted about spreading worldwide is one reason I don't beleive this would be used. Surely, even Allah must look poorly upon Muslims who kill their own children.

[Jorge]

You also assume the epdemic does not spread to parts of the world where it cannot be controlled through quarantines.

In that case we are no longer talking about "millions" of casualties in the U.S. which pretty much makes the poster's original point.
Besides, the idea that the epidemic would spread to other parts of the world much less prepared to deal with it than the U.S. especially third world nations, makes it less likely it would be the attack of choice.