California saw an almost 44 percent decrease in confirmed human West Nile virus cases in 2016, but there's a catch.

Sacramento-Yolo Mosquito & Vector Control District manager Gary Goodman said confirmed cases are underreported.

"The typical symptoms for West Nile tend to be similar to the flu," Goodman said.

Moreover, only 1 in 5 people who develop the virus actually feel the symptoms, which include fever, headaches and a possible pain around the eyes.

And it is not as simple as a mosquito bite. Goodman said it takes the virus five to 10 days to circulate.

He cautions against standing water and making sure water doesn't roll off into the street when watering grass.

The target mosquito the district is most concerned about travels between 1 to 5 miles as well.

West Nile tends to ramp up when it gets warmer. While there are no confirmed cases so far this year, Goodman expects human cases to start trickling in this June.