Some residents are wary of the "fogging" spray, but Carlos Sanabria with the Mosquito District says the pesticide Pyrethrian is a lot safer than contracting West Nile.

"We aren't applying it unnecessarily, and there is a threat to human health, so we take this decision very seriously," said Sanabria. District spokeswoman Deborah Bass urges everyone to cooperate, and if a neighbor isn't getting the message, she suggests, turning him or her in.

"One neglected or abandoned swimming pool can breed hundreds of thousands, if not a million, mosquitoes," said Bass.

Contra Costa County has seen 11 cases of West Nile over the past two years, two of them fatal.

KCBS' Tim Ryan reports