Smallpox was eradicated in 1977. Routine vaccinations stopped in the US in 1972. More than 40% of Americans and many others around the world have not been vaccinated and are vulnerable. Those who were vaccinated only once are also thought vulnerable.
Smallpox is contagious It spreads from person to person through the air and kills about 30% of unvaccinated persons. Should smallpox be reintroduced, it could spread throughout the world.
There has been concern about smallpox as a possible weapon for bioterrorism. The General Accounting Office, the investigative arm of the US Congress, in a 1999 report considered smallpox to be a "questionable" biologic threat for terrorism, because of the limited availability of the agent and difficulty processing it. However, the virus is very stable. Its lethal effects were deemed by the GAO to be "moderate to high" and the consequences of an attack are "deemed especially serious."
For more, please read Facts About Smallpox - Bioterrorism.