From a Washington Post article:
You know, if I was a small country or terrorist organization, and I was thinking about unleasing smallpox against the Western world, I sure as hell would not release a stock version of the virus. I would have my scientists working around the clock to genetically modify the original smallpox so that it's just as dealy, if not even more so, but that the standard vaccine would be worthless against it. Why would I waste time preparing a biological attack that my enemy can put up safeguards against?
And yes, sometimes it scares me that I think of these things. And sometimes it scares me that other people don't.