West Nile virus activity is on the rise in some northern California counties, according to the Sacramento-Yolo Mosquito and Vector Control District. Three dead birds and 13 mosquito samples tested positive for the virus on Aug. 13, “the highest number of positive results reported in a single day this year,” the agency said in a news release.

Northern California’s first human West Nile virus case of the year has been confirmed in Butte County, according to local health officials.

Most people who contract the West Nile Virus, don’t even know they have it. “Maybe about 70% of individuals who actually are infected, they have no real symptoms or response,” Ryan Smith, leader of the Iowa State University Mosquito Surveillance Program said.