Many biological agents could be used to attack people, but only a few, like the smallpox virus, have the ability to cause illness or panic and public health systems would be overwhelmed, according to the Centers for Disease Control and Prevention.

        The CDC released an updated smallpox response plan in late November, much of which was used 30 or 40 years ago to control outbreaks.

        If an outbreak occurred in the United States, three factors could contribute to a rapid spread of the disease:

        • The U.S. population has virtually no immunity to smallpox because the disease does not naturally occur anymore and routine vaccination was discontinued in the early 1970s.

        • Health officials may not recognize the disease.

        • Increased crowding in cities and citizens' mobility.

        Efforts have begun to expand the nation's vaccine stockpile from about 15 million doses now to more than 300 million, but the CDC's response plan will not include advance vaccination. That is because the risk of side effects outweighs the risk of someone actually being exposed to the disease.

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