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A Southern California city is being sprayed with pesticide after an increase in mosquitoes testing positive for the West Nile virus.

The Orange County Register reports vector control workers began spraying pesticide from trucks in Fullerton early Thursday and were to continue into Saturday.

Between Aug. 24 and Sept. 28, there were 29 mosquito samples that tested positive for the virus, including 27 from the area being sprayed. One of two human cases in Fullerton this year occurred in that area.

The pesticide is being applied at the rate of under an ounce per acre.

The Centers for Disease Control says most people infected with West Nile don’t have symptoms but about one in five develop a fever or other symptoms, and one in 150 develop a serious and sometimes fatal illness.