The woman, who lives in the eastern part of the county, was confirmed to have the virus by a state laboratory in Richmond on Thursday, according to the Contra Costa Mosquito and Vector Control District.
She is expected to make a full recovery.
There were four confirmed human cases of West Nile virus in 2010. No one in the county has died as a result of the disease since 2006, officials said.
The primary symptoms of West Nile virus are fever, head and body aches, nausea and vomiting, swollen lymph nodes and skin rash, according to the district.
People can protect themselves from the virus by eliminating mosquito breeding grounds, getting rid of stagnant water, wearing insect repellent containing DEET, Picaridin or oil of lemon eucalyptus, and reporting neglected swimming pools.
People can call (925) 771-6195 to report neglected pools and (877) 968-2473 to report dead birds, which are a warning sign indicating the presence of the virus.