Quite often you see people who have a round scar on their upper arm about the size of a nickle. If I'm not mistaken it's from a vaccination for smallpox, something that is no longer done. It's no longer done because smallpox has been stopped - people don't get the disease anymore. It's a fact that we frequently take for granted today, but wiping out smallpox was an enormous accomplishment.
In the history of the world it is estimated that smallpox has killed more people than any other events (like wars) or disease (with the possible exception of the Black Plague). The variola virus only aflicts humans and is extremely contagious and kills between 10 and 30 percent of its victims. There are some variations which are nearly 100% fatal, but they're so disgusting and horrific I'll skip describing them (Jamie got really mad at me when I read aloud to her while she was eating, but it was fascinating!).
But there's a happy ending to the story. In 1796 an English physician named Edward Jenner developed a vaccine by using the virus that causes cowpox. In the 1950s efforts to vaccinate people began to show signs of success although 2 million people a year were still dying from the disesase. In 1966 a worldwide eradication campaign led by Donald A Henderson and supported by the CDC and the WHO began sending teams to vaccinate people wherever an outbreak occurred. The hope was to create a ring around each outbreak and stop it from spreading uncontrollably. It was pronounced successful in 1980.
Surprisingly for a time of Cold War tensions, a great deal of vaccine and support for the campaign came from the Soviet Union. After it was eliminated, the virus was supposedly kept only in 2 freezers - one in Atlanta, Georgia, and another in Siberia, in the USSR. But as the date neared for destroying the remaining samples of the virus in the late 1990s, some began to object and word got out that the Soviets had been producing huge amounts of weaponized smallpox. Soviet defectors told of progams that had produced tons of bioengineered and genetically modified virus, and other evidence tended to confirm the news. The Soviets denied such work, but experts say it's wishful thinking to believe the virus is still only in those 2 freezers - especially following the breakup of the USSR in the early 1990s.
Two excellent books detail the history and dangers posed by smallpox today. Jonathan B. Tucker calls smallpox "the world's most dangerous prisoner" in his book Scourge: The Once and Future Threat of Smallpox which focuses more on the inspirational erradication of the virus but also covers the potential threats from known and unknown stockpiles. The Demon in the Freezer: A True Story by Richard Preston covers much of the same information but starts with the anthrax deaths shortly after the 9/11 terrorist attacks, and the potential for biological weapons (not just smallpox but also anthrax and ebola) is covered in more depth. Both books are excellent - Scourge is perhaps more informative while Demon is slightly more readable - but the only downside is that both may be a bit dated by now - Scourge was published in 2001 and Demon in 2003. Nonetheless, they're both books I highly recommend.
And those smallpox vaccinations? They only provided immunity for about 5 to 10 years. Think about that!