Just to remind everyone what we are dealing with here: http://en.wikipedia.org/wiki/History_of_smallpox

  During the 20th century, it is estimated that smallpox 
  was responsible for 300–500 million deaths.

Half a billion people dead, after a vaccine was discovered. And that was one century of the 12,000 years it has existed.

That's why I get shivers from the opening of http://en.wikipedia.org/wiki/Smallpox:

  Smallpox *was* an infectious disease...

  [...]

  To this day, smallpox is the only human infectious
  disease to have been completely eradicated.