The infected bird is the first indication that the virus is circulating in the area, according to the district. Birds host the virus, which is then transmitted through infected mosquitoes, officials said.
The bird in South San Francisco was found May 15, and vector control officials will now chart reports of dead birds to determine areas of risk. Last year, two dead birds and one squirrel in the cities of San Carlos and San Mateo tested positive.
Residents are asked to report dead birds online at www.westnile.ca.gov or by phone at (877) WNV-BIRD.
Vector control officials also ask that residents drain standing water to prevent mosquitoes from laying eggs, wear long sleeves and pants and limit outdoor activity during dusk and dawn hours when mosquitoes are active, and protect against bites with bug spray.
Mosquito problems can be reported to the vector control district at (650) 344-8592.