The woman became sick in mid-July, but wasn't diagnosed with the virus until late July, public health nurse Susan Farley said.
The woman is recovering from her illness, but details about the severity of her case were not being released, Farley said.
Farley said the county health department is urging people to wear insect repellent when they are going to be exposed to mosquitoes and to take steps to reduce potential exposure, including making sure their window and door screens are in good shape and draining sources of standing water where mosquitoes can breed.
Deborah Bass, spokeswoman for the Contra Costa County Mosquito and Vector Control District, is also urging people who find dead birds to report them as soon as possible.
Dead bird reports are entered into a statewide database that allows local mosquito control districts to track where the virus is most prevalent and therefore where they are most likely to see human cases, Bass said.
People who find a dead bird can call the dead bird hotline at (877) WNV-BIRD or (877) 968-2473.
So far this year, 14 groups of mosquitoes, 13 dead birds and one sentinel chicken have tested positive for the virus in Contra Costa County, Bass said.
In 2008, five people from Antioch, Brentwood, Oakley and Martinez tested positive for the virus. All five patients survived the illness, according the health department.
So far this year, there have five human cases of the virus in California, including the one in Contra Costa County, two in Kern County, one in Orange County and one in San Diego County. At this time last year, there had been 28 human cases in California, according to the California West Nile Virus Web site.