The mosquito sample was collected in the area of West Cypress Road and Ohara Avenue, Contra Costa Mosquito and Vector Control District spokeswoman Deborah Bass said.
August is typically the time of year when people are most likely to become infected with the virus, and the mosquito district is urging residents to take precautions, Bass said.
People should remove any standing water around their homes to reduce mosquito populations and wear insect repellent containing DEET, Picaridin or oil of lemon eucalyptus.
Residents are also urged to report abandoned swimming pools, which can produce up to 1 million mosquitoes that can infect people up to five miles away, Bass said.
People can call (925) 771-6195 to report abandoned pools. Anonymous calls are accepted.
"We just want to know where they are so we can take care of them," Bass said.
The district is also asking people to report dead birds by calling the state's West Nile virus hotline at (877) 968-2473.
The mosquito district will often test dead birds for the virus, but even if they don't test the birds, the reports are entered into a database that helps the district identify clusters of dead birds, Bass said.
"That information tells us we need to concentrate our control efforts in a specific area so we can protect people from the virus," Bass said.
She said that so far this year dead bird reports are down from previous years.
While studies show that the majority of West Nile virus infections in humans go undiagnosed, 26 people in the county have been diagnosed with the illness since 2005, including two who died from it in 2006, Bass said.