The West Nile virus has resurfaced in Contra Costa County, according to the county's Mosquito and Vector Control District.
A dead American crow recently found near the intersection of Danville Boulevard and Camille Place in Alamo tested positive for the disease, district officials said.
"This bird is one of the earliest signs of the West Nile virus we've had in a season in the county to date," said Steve Schutz, the district's scientific program manager.
Since 2005, when the virus first surfaced in Contra Costa County, there have only been two earlier starts to the West Nile virus season, according to district spokeswoman Deborah Bass.
District officials say the earlier advent of the virus season isn't surprising since the unusually warm winter created ideal conditions for mosquitoes to thrive.
"Unfortunately, the earlier West Nile virus is detected, the earlier we may see human cases," Schutz said.
Many people who become infected with the virus show no symptoms, while others exhibit flu-like symptoms including fever, head and body aches, diarrhea or a rash.
In 2012, the last year for which there are records, roughly 212 Contra Costa County residents were infected with the disease, according to Bass.
She said the first line of defense against mosquitoes carrying the virus is to dump any standing water.
Residents are also advised to avoid being outdoors when mosquitoes are present, usually at dawn or dusk and to wear mosquito repellants while outdoors.
The public is urged to report dead birds, which can then be tested for the virus, to the state hotline at (877) 968-2473 or at www.westnile.ca.gov.
Neglected swimming pools, a major source for virus-carrying mosquitoes, can be reported at (925) 771-6195.