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The park sent warning emails and letters Wednesday to another 1,000 people who stayed in tent cabins, after officials found that a computer glitch had stopped the notices from going out with the original 1,700 warnings Monday. The warning says anyone with flu-like symptoms or respiratory problems should seek immediate medical attention.

In 2011, half of the 24 U.S. hantavirus cases ended in death. But since 1993, when the virus first was identified, the average death rate is 36.39 percent, according to the Centers for Disease Control.

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Dearen reported from San Francisco.