West Nile virus transmission cycle: When a mosquito bites an infected bird, the virus enters the mosquito's bloodstream and eventually moves into its salivary glands. When an infected mosquito bites an animal or a human (host), the virus is passed into the host's bloodstream, where it may cause serious illness. (Photo courtesy of Mayo Clinic)
A small percentage of California’s West Nile virus cases were reported in the Bay Area over the last 13 years. 

In that time period, 130 cases of 5,300 statewide were reported in the Bay Area, with the highest number in Contra Costa County, according to the California Department of Public Health. The disease, which can be deadly to humans, is mainly found in birds. Humans can get it by being bitten by a mosquito carrying the virus. 

Only one case of a person acquiring the disease has been reported in the Bay Area so far in 2019. That was in Solano County, and that person survived. Since 2006, the West Nile virus has killed 303 people in the state. 

Here’s how the counties rank by the number of cases from 2006 to Oct. 31, 2019.1. Contra Costa County — 54 2. Santa Clara County — 333. Solano County — 234. Alameda County — 75. Marin County — 46. Napa County — 47. San Francisco — 48. Sonoma County — 19. San Mateo County — 0 Nola Woods, spokeswoman for the Contra Costa Mosquito & Vector Control District, said climate change and increased travel mean vigilance is important in fighting the West Nile disease.

“We must keep our eyes open for not only sources of water that can produce mosquitoes, but new mosquito species that are finding their way to areas where they were not previously established,” she said.

SourceCalifornia Department of Public Health; Contra Costa Mosquito & Vector Control District