Rodent control is the primary strategy for preventing the hantavirus syndrome, according to the CDC.

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Yosemite National Park has increased routine measures to reduce the hantavirus risk, according to officials.

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The two recent hantavirus cases bring the 2012 total in California to four.

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The visitors contracted hantavirus cardiopulmonary syndrome, a rare but often fatal lung disease.

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It's best not to vacuum or sweep, because that can push hantavirus particles into the air, where they can enter the lungs.

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Yosemite National Park saw one hantavirus case each in 2000 and 2010.

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There is no specific treatment for hantavirus infection, according to the CDC, but the earlier the patient is brought to intensive care, the better.

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In the United States, the carriers of hantavirus are deer mice, cotton rats, rice rats and white-footed mice, according to the U.S. Centers for Disease Control and Prevention.

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It comes after two rodent owners were infected with the hantavirus, which can be passed to humans through the animals' urine, faeces and saliva - usually through a bite.

BBC: Rat infection: North Wales owner catches hantavirus