PIERRE - The South Dakota Health Department has confirmed 15 human cases of West Nile disease so far this season.

That's an increase of four from last week.

Infections have been found in 12 counties. They include Minnehaha, Lawrence, Sully, Beadle, Hughes, Stanley, Corson, Brown, Davison, Lincoln, Charles Mix, and Union counties.

Brown County has the most confirmed cases of the disease: four.

The West Nile virus is transmitted to humans by mosquitoes.

Last year, 208 people were infected and six died in South Dakota.

One-third of those infected this year have required hospitalization. None of them died.