Officials say the patient was an elderly person living in the city of Merced and they will not be identified for privacy reasons.
Experts say seniors are often considered to be at higher risk if infected.
West Nile Virus is spread through the bite of an infected mosquito.
Mild symptoms include fever, headaches, body aches, nausea, and vomiting.
There is no specific treatment for a West Nile Virus infection.