Same way they got an obviously weaponized anthrax.
Do some reading on biological warfare. Small pox is still out there as a weapon.
I've read some on the now dismantled Soviet biological weapons program, and some on the US weapons program. I haven't seen much on what the Iraqis are doing, nor much on the modern Russians. The sense I get is that smallpox was considered hard to weaponize on any mass scale, too survivable, and too easy to contain. That's why others, such as anthrax, gained ascendency in weapons programs -- easier to weaponize (still not easy), and less survivable. Anthrax may yet prove to be fairly easily contained (in any sense of a mass infection). Normal hygeine (wash hands before eating, don't pick your nose) and a little more alertness to the unusual should protect the average person.
So far all I see are a few selected high profile infections. IMO, we are more at risk of panic than a pandemic.