The virus was also detected in three dead crows found in Orland and Willows.
And in Butte County, a "sentinel chicken" tested positive for the virus.
These instances represent the first time this year that the virus has appeared in the north valley.
The woman who tested positive didn't become ill, said Scott Gruendl, director of the Glenn County Health Services Agency.
It was discovered she had the virus after she donated blood at a Chico blood bank, Gruendl said. The blood bank found signs of the virus when it tested the woman's blood.
Gruendl said the blood bank notified county and state health officials. It was determined the woman had been infected fairly recently — probably within the last several weeks.
Jack Cavier, manager of the Glenn County Mosquito and Vector Control District, said his agency has stepped up efforts to reduce the mosquito population in the Orland and Willows areas.
On Tuesday, the district sprayed pesticides in Orland neighborhoods, and a similar operation was planned in Willows this evening. District staff have also been searching for places mosquitoes might breed, such as in untended backyard ponds.
Butte County's mosquito-control district has also begun fogging in the southern part of the county.
West Nile virus is spread by certain types of mosquitoes. Most people who contract the disease
The Butte County Mosquito and Vector Control District keeps flocks of chickens and tests their blood every two weeks to see if they have any vector-borne diseases.
Recently, one of the district's chickens tested positive for West Nile, said Matt Ball, the district's general manager. It is part of a flock kept on the Butte County side of the Sacramento River across from Hamilton City.
Butte and Glenn counties are now the farthest north the virus has been found this year in California, according to the  website maintained by the state Department of Public Health.
So far this year, according to that site, there have been seven human cases of the disease in five counties. On Thursday, the site did not yet show the case from Glenn County.
A news release from the Glenn County Mosquito and Vector Control District offered the following tips on how to avoid contracting West Nile virus:Eliminate all sources of standing water on your property. Avoid being outside when mosquitoes are most active, especially at dawn and dusk. If you are out at dawn or dusk, wear long pants and a long-sleeved shirt. Use insect repellent containing DEET, Picaridin or oil of lemon eucalyptus. Make sure your doors and windows have tight-fitting screens. Repair or replace screens that have tears or holes. Contact your local mosquito and vector-control agency to report neglected swimming pools or if there is a significant mosquito problem where you live or work.