Hey, it's certainly possible that the Soviets did it another way, or discovered the same way separately. But I'm just going off what the smallpox experts said when they were discussion weaponized smallpox on NPR last week.

But as far as the CDC is concerned, the last case of smallpox was in 1978, and lab-aquired.

(Pause for quick Google search)

Aaaahhhh... it's hypothesized that a 1971 outbreak of smallpox was possibly due to the testing of a biological weapon, a virulent aerosolized version. But not necessarily GE, possibly a strain selected for it's virulence. (Aside -- the Russians have some nasty naturally occuring versions of these killer diseases. Their version of anthrax is much worse than the version found naturally in the US.)   Interesting article here:
Radio Free Europe