Some 176 species of mosquitoes are recognized in America, but only one -- the culex -- actually carries West Nile virus.
"We run it through the testing the same day we collect, and we get results," said Desiree Keeney, field operations manager.
And test results this week show West Nile has returned to Ada County about the same time it showed up last year.
The mosquitos infected with West Nile virus were found around Beacon Light and Linder roads. Now the county has this area under increased observation.
The county will target larvae in standing water and spray anti-mosquito fog at night and residents are urged to take precautions.
"Wear DEET repellent, and long sleeved clothing," said Keeney. "Cover yourself if you're going to be out at dusk and dawn."
In 2006, Idaho led the nation in West Nile virus infections and 23 people died.