West Nile virus was found in mosquitoes caught in traps in Keating Valley and near Haines on July 25.
This is the first confirmed presence of the virus in the county this year, and the first since 2009.
Thus far the virus has not been found in birds, horses or people.
West Nile virus can be transmitted from mosquitoes to animals.
Officials from the Baker Valley Vector Control District, in addition to trapping and testing mosquitoes across the 200,000-acre district that includes most of Baker, Bowen and Keating valleys, is looking for dead birds that might have been killed by the virus.
If you find a dead crow, magpie, jay or robin, call the district at 541-523-1151.
Most people who are infected with the virus don't have any symptoms, but the infection can cause illness and, in rare cases, prove fatal.
The state has set up a West Nile virus hotline at 1-800-702-INFO.