About four or five people in Contra Costa County have been diagnosed with West Nile virus every year since 2006, said Deborah Bass, a spokeswoman for the Contra Costa Mosquito & Vector Control District.
The virus is in the birds, and mosquitoes bite the birds and then spread West Nile to humans, Bass said.
"Every year when we learn that the birds have it, it tells us where the virus is active," she said. "This year it happens to be in Walnut Creek, so we know we need to step up our efforts in that area."
The district advises residents and visitors of Contra Costa County, and especially Walnut Creek, to wear insect repellent that contains DEET, Picaridin or oil of lemon eucalyptus, and to dump or drain standing water.
The public is also advised to avoid being outside at dusk or dawn and to report neglected swimming pools, which can each breed more than 1 million mosquitoes that fly up to five miles away, Bass said.
She said that although the county only has a few confirmed human cases of West Nile virus each year, many more people are infected than the official diagnoses suggest.
"Often they don't feel sick enough to go to a doctor, of they might write it off as the flu or not feeling well," Bass said.
About 80 percent of people infected by the virus don't experience any symptoms, she said.
Most of the remaining 20 percent experience varying degrees of sickness and may or may not decide to go to the doctor, but about 1 percent of cases prove fatal.
Those 1 percent of patients experience encephalitis, which is swelling of the brain, Bass said. Six people died in California last year from West Nile virus out of 111 confirmed cases.
Warm weather speeds up the virus lifecycle, so it's especially important for people planning to spend this Fourth of July weekend outdoors to wear repellant and drain standing water, Bass said.
Highs are expected to reach the 80s and 90s inland this weekend, according to the National Weather Service.
"It only takes one bite for someone to get infected and have a serious problem," Bass said.
Neglected swimming pools and other mosquito problems can be reported to the district at (925) 771-6195.