Seven months later, they put a version of the AIDS virus that affects monkeys in the animals' rectums.

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Just as a virus that affects humans can mutate and confound medical experts, each version of Sasser -- labeled A, B, C, and D -- poses slightly different challenges for information technology experts.

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The chronic version of the condition -- which is defined as lasting for 12 weeks or more -- affects about 1 in 6 adults in the U.S., according to the study.

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