Slate wiper if they can pop it in enough places to prevent firewalling it.
I'd guess that if small pox broke out among Chinese or Russian troops at the front, their parent governments would nuke them from along with reducing North Korea to glowing cinders.
Have it introduced in the midst of a Twilight nuclear exchange and the human race will simply die off until population density is too low to let it spread effectively. Then we'll just have endemic occasional outbreaks that take 10% here or 20% there in bad years.
I always wanted to run a T2K game where players were somewhere near-ish Central Asia (Iran, or maybe a Russian game set in the Rodina) where characters had to deal with some roving neo-nomads who got exposed to something nasty while looting a Soviet bioweapons lab and were now carrying Small Pox or genetically modified Plague or something equally foul.