A 65-year-old woman from the eastern part of the county recently infected with the virus is now recovering, according to Contra Costa Health Services.
In addition, a bird found in Concord has tested positive for the disease.
Vector control technicians are working to eradicate mosquitoes throughout the county that are spreading the disease.
Since 2005, some 40 county residents have been diagnosed with the virus -- two of whom died in 2006.
Contra Costa Mosquito and Vector Control District spokeswoman Deborah Bass said the virus is "endemic throughout the county" and can be tough to root out, as many disease-causing mosquitoes are found in residents' yards.
Still, there are a number of measures residents can take to help prevent new West Nile cases in the area.
"Dumping water out and maintaining swimming pools would go a very long way in the fight against mosquitoes and the diseases they transmit," Bass said.
County vector control officials said residents can also defend themselves against the disease by using insect repellent and avoiding being outdoors when mosquitoes are present -- especially at dawn and dusk.
Residents are also urged to report dead birds by calling the state's West Nile virus hotline at (877) 968-2473 and to report neglected swimming pools at (925) 771-6195.