No. It said an atomic bomb OR a missile carrying smallpox. Not all missiles are nuclear. Of course, with smallpox there is no need for anything so complicated as a missile, since an infected blanket will do. The scabs from an infected person, such as a prisoner who has been deliberately infected to produce enough contagious material, can be delivered any number of ways.
Anyone who tried that would have to be extremely desperate and willing to lose it all, because the reaction would be huge.
Of course. For want of a comma....