Prior to the mid-twentieth century, smallpox was one of the most dreaded diseases. Even if you recovered, your face and body were covered with pockmarks for the rest of your life. It was extremely infectious from one human to another and one of the most lethal diseases in circulation. Vaccines were first introduced against smallpox, and they were enormously effective almost from the very beginning. Vaccinia virus is currently used to make vaccine against variola virus, the virus that causes human smallpox. Except in very rare cases, the vaccinia-derived vaccine has no adverse reactions in humans. Because poxviruses tend to be very host-specific, there is no smallpox reservoir other than what exists in human hosts. Realizing this, immunologists organized a worldwide organization with the intention of completely eradicating smallpox. All they needed was a very large supply of effective vaccine and many field workers.
Area by area, smallpox was eliminated. If a case of smallpox was spotted, every effort was made to isolate the victim and vaccinate everyone who might have come into contact with him or her. By 1977 it appeared that smallpox had been eradicated worldwide, and no new cases have been reported since. The Soviet Union and the United States, however, held onto supplies of smallpox for military, defense, and research purposes. There is a fear that these stocks may have been pilfered and that there may be others—including terrorist groups— in possession of smallpox viruses. Because most people have not been vaccinated against smallpox for more than a quarter of a century, the possibility of uncontrolled possession makes smallpox an extremely serious threat, perhaps even more serious than it was when vaccination against smallpox was a common practice.