From sundown to 11 p.m., crews will fog the area around Brentwood where infected mosquitoes and birds were discovered. Health officials caution people to stay indoors and turn off the air conditioning as a precaution.
The black crow found Thursday is the 23rd bird discovered with the disease. The infection comes from two specific types of mosquitoes.
A batch of mosquitoes tested Thursday by Contra Costa Vector and Mosquito Control was the tenth group found to carry the virus this year.
Dr. Steve Schutz, the lab's scientific programs manager, says the increased mosquito activity this year is due to the mild winter weather.
It's likely that more mosquitoes than usual survived the winter, according to Schutz. "It's those mosquitoes that emerge from their winter hibernation that seem to start the cycle up every year, so it started kind of early this year," he said.
Most people who are infected with West Nile suffer flu-like symptoms, but for others it can be deadly. In 2006 two people in the county died from the disease.
When high levels of mosquitoes are detected, an insecticide derived from chrysanthemums is sprayed.
"It's toxic to mosquitoes because mosquitoes are so tiny, so an extremely tiny dose of that material will kill a mosquito but the amount that we use is way too little to cause any harm to people or birds or other animals," said Schutz.
Mosquitoes breed in even small amounts of water, and experts say getting rid of breeding sites would go a long way in attacking West Nile virus.
Neglected swimming polls are a huge issue, especially in eastern Contra Costa County, according to Deborah Bass of Contra Costa Mosquito and Vector Control.
"One neglected swimming pool can produce more than one million mosquitoes and infect people up to five miles away," she said.