Infected mosquitoes had been found in Ladera on July 18 and in the San Mateo area, officials said.
While birds are the primary hosts, humans, horses and other animals can be infected. Among human victims, 80 percent show no symptoms at all, officials say.
Infection requires the bite of an infected mosquito; the virus cannot be spread person-to-person.
Go to westnile.ca.gov for more information.
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