The enemy attacked yesterday with stealth, speed and stunning success. By afternoon, an untold number of people across the Washington region were in increasing agony -- sneezing, sniffling, rubbing their eyes and gasping for help.

Their loved ones could do little but hand them the antihistamine.

The nation's terror threat alert has again been lowered to yellow -- coincidentally the same color as the most potent biological weapon around for springtime hay fever sufferers. Yesterday, with the sun and warm weather aiding the assault, pollen struck hard for the first time this year. And those who fear it knew it.

Oaks led the way, backed by the region's significant forces of ashes, birches and sycamores. Together, they pushed the count for trees -- measured in terms of pollen grains per cubic meter of air -- to 1,597, a number at the extreme end of any allergist's scale. Only two weeks ago, it had been a barely irritating 526.

"When the oaks start, those numbers start tripling," said microbiologist Susan Kosisky. Every morning, Kosisky climbs to the roof of a two-story building at the Walter Reed Army Medical Center's Forest Glen annex to retrieve two small metal rods that have been collecting grains and spores in the air for 24 hours. Then she heads downstairs to the allergen extract laboratory, where she begins the painstaking task of counting.

This time of year, she resorts to extrapolating. "By the end of the morning, you don't want to see another pollen grain," she said.

Yet the battle has just begun. Allergists say 2003 is likely to be the worst in years because of this spring's heavy rain. "High-moisture years make it a terrible season," said physician J. Gordon Vap of McLean. "Though I hope I'm wrong. You don't want to see all these people sick."

Sick, as in a common cold, would be easier. It would mean quicker relief. Those afflicted with "seasonal allergic rhinitis" -- which includes one in five Americans, according to the American Academy of Allergy, Asthma and Immunology -- can sniffle through the entire summer as tree pollens give way to grass and weed pollens and mold spores multiply into the thousands.

"A lot of people look at it as if they have a defect," Vap said.

For more than a millennium, people throughout the world have suffered similarly. The earliest account of symptoms dates to the year 865, when Persian physician Mohammed Al Razi wrote a report on his philosophy teacher, who complained annually of sinus pain and inflammation "when the smell of flowers amplifies the illness." According to his manuscript, the recommended treatment was extreme: In cases of severe nasal pressure, Al Razi wrote, the patient's hair should be cut and the head covered with mustard.

Seven centuries later, an Italian physician described how the scent of roses could set off sneezing, an itchy nose and headaches. The first detailed account of seasonal allergies came in 1819 from an English physician, John Bostock, who studied his own affliction. Bostock is credited with coining the phrase "hay fever" because his condition typically worsened during haying season. Neither hay nor fever come into play for most people, however.

What does tends to feel much worse. "Red, puffy, swollen, itchy eyes," physician Martha White said yesterday. "I think this is going to be eye season. I'm seeing a lot of people" with that.

White practices with the Washington Hospital Center's Institute for Asthma and Allergy. She has two decades of research and clinical practice. But like Bostock, she also has personal experience. Faced with "trees, dust, ragweed and animals," she takes shots to stay sneeze-free.

The Washington area is an allergist's dream -- or nightmare. Although people are born with a predisposition toward hay fever, it takes a combination of factors to start suffering. "It takes the right genes and the right amount of exposure," White explained. And in the nation's swampy capital, "we have year-round mold."

Ah-choo. Pass the tissue.