PONTIAC, Mich. (CBS DETROIT/ AP) — A Detroit-area resident who donated blood has tested positive for the mosquito-borne West Nile virus.

West Nile virus primarily is transmitted to humans through the bites of infected mosquitoes. Mosquitoes are infected with the virus by biting infected birds.

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The Oakland County health department has been notified by state health officials and says it is the first blood donation in 2019 that has tested positive for the virus in Oakland County.

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Older people are vulnerable, especially if they have other health problems. Most people who become infected will not develop any symptoms, but some become sick three to 15 days after exposure.

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