Except it wasn't eradicated. It's still stored at the US's Fort Detrick, and in Russian and Chinese bioweapons facilities, too be released as a bioweapon, now that no one has natural immunity anymore.
Most people aren't vaccinated for smallpox, and only a very small amount of vaccine is even produced, mostly for medical and military people. Targetting is easy, here's your blanket while the power is out...