When the big black crows started dropping dead out of the sky, littering the highways and Little League fields by the dozens, the phones at the San Bernardino County Vector Control office started ringing.

More than 800 birds, mostly crows and ravens, have turned up dead across the county since April. An indicator the mosquito-borne West Nile virus has arrived in the area, the dead birds have become so common that county officials have stopped picking them up for testing, telling residents to dispose of the birds themselves.

"We saw one drop right out of the sky. It was flying along and all of a sudden, boom. There it was. I'm scared to death. If it can kill a horse, it's something to be worried about," said Marilynn Edginton, 61, as she hitched up the cuff of her light-blue capri pants to reveal a red welt on her calf and gave the mosquito bite a quick scratch. "Something bit me the other day and I even used Off."

The Fontana homemaker now keeps her teenage granddaughters inside after dusk and douses them with bug spray before they can leave the mobile home they share.

For the past five years, the vector control manager, Joan Mulcare, has sat in her San Bernardino office surrounded by maps and charts, gearing up for this. West Nile has steadily made its way across the country since the first case was reported in New York in 1999, and Mulcare knew it was just a matter of time before the disease crossed the California border and into her territory. Twenty human cases have been reported in the state already this year -- the majority of them here in San Bernardino County.

More than 100 human cases have already been reported across the country, well ahead of the pace of the past two years. While Maryland, Virginia and the District have seen little activity this year, the West is being hit hard. Arizona has had the worst outbreak so far with 66 cases. Three people -- two from Arizona and one from Iowa -- have died.

With long, hot summers and the presence of two different mosquitoes known to be particularly effective carriers of the virus, western states are particularly vulnerable to the disease. Last year, the West Nile virus killed 264 people and sickened nearly 10,000 others in the United States. California reported just three human cases, including one woman who apparently contracted the disease out of state. But this year looks to be different as birds have finally brought the disease into the populated, urban oases of the West.

"It's like a wildfire. You can see it burn progressively towards you, closer and closer," said Wakoli Wekesa, a vector ecologist for San Bernardino County.

In most cases, people who are infected never become sick or have only mild flulike symptoms. But the elderly, the very young and people with compromised immune systems are more susceptible. In rare cases the virus can cause encephalitis or death.

Health officials here are hoping the five-year head start they have on preparing to combat the disease will help to soften the blow, but they know they are in for a fight. San Bernardino County -- the largest county in the country -- covers more than 20,000 square miles. And officials have more to worry about than just residents. The county is home to some of the region's most popular summer destinations, from Lake Arrowhead and the campgrounds of Big Bear Mountain to Joshua Tree National Park and the Colorado River. Campers and water sports enthusiasts descend on the area in droves, enjoying the warm weather and outdoor barbecues. There are 82 summer camps.

Authorities are urging residents to use insect repellent and stay indoors at dusk and dawn, when mosquitoes are most active.

Ornamental ponds, plastic wading pools, and even outdoor water dishes for dogs and cats can be potential havens for mosquitoes, which need water to lay their eggs. Sheriff's department helicopters are flying high above neighborhoods, looking for dirty backyard pools and snapping aerial surveillance photos. Residents with unkempt pools and standing water on their property receive a visit from county officials, who hand out information about the disease and instructions on draining the standing water. Violators can be fined as much as $1,000 a day.

At the Wal-Mart in San Bernardino, storewide announcements remind shoppers of the dangers posed by the disease and direct them to the rows of insect repellent.

There are public service announcements on local radio stations. Local drugstores are distributing fliers about the disease, and pharmacists are on hand to answer questions.

The virus is not going away anytime soon. New England reported more cases last year than in any previous year, and health experts expect the virus to continue to be a threat nationwide.

More concerned about the threat of another summer of wildfires, Mark Strain of Highland still is not willing to take any chances, cutting back on his nightly two-mile walks with his dog, leaving her to use the doggie door instead.

"She really enjoyed those walks," Strain said of his dog. "That's how she kept up with the other neighborhood dogs. But then people always think, hey, that couldn't happen to me."

Harley, a barred owl, gets a vaccination for the West Nile virus in Omaha.