CONTRA COSTA COUNTY — A third dead bird and a third group of mosquitoes have tested positive for West Nile virus, the Contra Costa Mosquito and Vector Control District said Friday. The discoveries mark the latest in a string of infected animals and insects across the Bay Area.

The bird was found in Oakley, while the mosquitos were trapped in Brentwood. Earlier in the day, officials in Napa County also said they had confirmed West Nile among mosquitos in Calistoga. The announcements come after Contra Costa County officials found the virus among two groups of mosquitos from Discovery Bay and two dead birds from Martinez and Brentwood last month.

“Statistically, this is the time of year when we typically see the highest risk of human cases,” Steve Schutz, the district’s scientific programs manager, said in a news release.

West Nile can be transmitted to animals and people through a mosquito bite. Typically, the virus only causes mild, flu-like symptoms, but older adults and those with weakened immune systems can have more severe reactions. It can occasionally cause meningoencephalitis, the inflammation of the brain and spinal cord, according to the Contra Costa Mosquito and Vector Control District.

The district said residents can protect themselves by wearing an insect repellent registered by the Environmental Protection Agency or those containing DEET, Picaridin, IR3535 or the repellent version of oil of lemon eucalyptus. Residents are also encouraged to dump out standing water, scrub any containers of standing water to remove mosquito eggs, and ensure door and window screens are working properly.

There is no vaccine or treatment for West Nile virus.

Contra Costa Mosquito and Vector Control District officials are now conducting increased surveillance in the areas where the bird and mosquitos were found.