In Mason County.
Here's the state Department of Health:
The Washington State Department of Health received confirmation today from the Centers for Disease Control and Prevention (CDC), that a Mason County man in his 20’s, who visited a Thurston County hospital, is the first person in the state to test positive for Zika virus. The person recently traveled to the South Pacific before returning to Washington. People who’ve returned from Zika-affected areas who are pregnant or having symptoms of Zika illness should contact their healthcare provider.
The good news is that "this virus spreads through the bite of a type of mosquito we don't have in Washington State, so it is very unlikely that this virus would spread widely here," Washington State epidemiologist Dr. Scott Lindquist said in a statement.
The DOH recommends that if you're traveling to a place with the type of mosquito that transmits Zika—the Aedes mosquito—you talk to your doctor.