His case was discovered July 25, the day he donated blood to a Blood Centers of the Pacific facility in Fairfield, blood center spokeswoman Lisa Bloch said.
The man, a regular donor, had no symptoms of the virus. His blood has been discarded and he must wait 12 months before he can donate again, Bloch said.
There have been 12 other human cases of the virus in California this year. Three cases were recorded in Stanislaus County and one in San Joaquin County.
Blood donation centers have been testing for west Nile virus since 2003, Bloch said.
Blood Centers of the Pacific supplies blood to 42 hospitals throughout the region. Bloch said the detection of the virus in the Vacaville donor attests to the effectiveness of the screening.
Blood is also screened for HIV, hepatitis B and C, syphilis and a parasite infection called Chagas disease.