No one can say for certain when the first rabid raccoon ambled through the Washington region, but biologists peg it at the late 1970s. From there, raccoon rabies blossomed through Virginia, Maryland and the District.

Today, if a rabid fox lashes out at a group of children, as happened this month in Herndon, chances are it got its rabies from a raccoon, biologists and public health officials say.

Household pets catch the deadly disease relatively rarely. But raccoons have become the carrier of choice -- and the target of a multimillion-dollar campaign to cover their habitat (virtually everywhere) with millions of matchbox-size, fish-scented blocks spiked with a two-milliliter pouch of liquid rabies vaccine.

In the Washington area, the effort began in 1998 in Anne Arundel County, which dispatches airplanes, helicopters and pickup trucks loaded with the vaccination biscuits. Nationally, a massive campaign is underway, dropping about 8 million of the vaccine treats from the Gulf of Mexico to Lake Erie.

The national effort aims to stanch the westward flow of raccoon rabies, which developed after the virus, once confined to Florida and Georgia, began to spread.

Anne Arundel is the eastern front of the battle, and this year Garrett County in Western Maryland became the western edge of the national campaign. The idea is to bring the two fronts together somewhere around the Washington area within the next few years and, ideally, eradicate the raccoon strain of the rabies virus entirely.

Dogs were once the most pernicious carriers of the disease. No longer, said Joseph Horman, public health veterinarian for the Anne Arundel Health Department.

"If you go back to the '40s and '50s, you had a dog rabies problem," Horman said. "That's not so true anymore. It all changed with raccoons."

Summer always brings a spike in reported cases of rabies. Human deaths are exceptionally rare in the United States. The last confirmed human rabies death in Maryland was in 1976; in Northern Virginia, a man died in a freak exposure in 2003. If contracted by humans, the disease can be fatal. Once symptoms develop, rabies is always fatal.

As suburbs move farther into rural areas and animal habitats shrink, however, reports of human exposure to rabies tend to increase. Biologists say that makes the effort at least to control raccoon rabies -- and possibly eradicate it -- an important insurance policy.

"When the raccoon rabies epidemic began [in the late 1970s], raccoons began to quickly dominate in terms of total number of rabies cases in the United States," said biologist Dennis Slate, national rabies coordinator for the U.S. Department of Agriculture. "The thing about raccoon rabies that's very problematic is that, unlike other rabies strains, it spills over into other species very easily."

The leap that took raccoon rabies from the South to West Virginia in a matter of years puzzles epidemiologists. Some speculate that people wanting to hunt them moved them from the South to the mountains of West Virginia. Others believe it might have been an effort by wildlife officials to bolster a local raccoon population in West Virginia with southern raccoons.

Horman leads the guerrilla vaccination effort in Anne Arundel, where reported cases of rabies spiked in the mid-'90s. Health officials began spreading the raccoon "bait" in the Annapolis area in 1998, and it seemed to put a dent in the problem.

They expanded the program gradually until the entire county was covered. This summer, Anne Arundel experienced a marked rise in reported cases of rabies. As of Aug. 10, the county had 18 confirmed cases of rabies in animals -- almost matching last year's total of 20. In Montgomery, Fairfax and Prince George's counties, the number of confirmed cases of rabies is about normal, officials said.

Last week, as helicopters loaded with thousands of pieces of bait lifted off from Tipton Airfield in Anne Arundel, biologist Jeremy J. Smith of the Agriculture Department drove a pickup loaded with several hundred of the bait pieces and tossed them out of the cab. More than 416 square miles -- the entire county -- will be covered with vaccine dropped from helicopters, planes and pickups. Officials say the baits don't pose a risk to humans or pets.

"We just saturate the areas where we expect raccoons to live," Smith said. "In other words, everywhere."

Volunteers Martie Pennington and Debbie McLean await the helicopter that will carry them over Anne Arundel County to drop rabies vaccine cubes.Fishy bait spiked with a liquid vaccine is part of a campaign to stop rabies in raccoons. The animals became major carriers of the disease in the 1970s.