Who hasn't suffered occasionally from a periodic, persistent "stomach flu"? Odds are, it could have been an intestinal disorder unrecognized a quarter-century ago but which is rampant in the United States today. Called giardiasis (pronounced jar-DYE-a-sis), it hits countless Americans each year, from toddlers in day-care centers to backpackers in the Rockies, bringing death to few but discomfort to many.

Although unfamiliar to most laymen and some physicians, giardiasis is the most common parasitic disorder in the U.S. today, reports the National Institutes of Health. It is spread by contact with feces containing a minute protozoan life form: Giardia lamblia.

Concerned over Giardia in public water supplies in the U.S., the Environmental Protection Agency has proposed new water purification rules requiring municipal water throughout the nation to be free of 99.9 percent of all Giardia. EPA began two days of hearings on the rules in Washington yesterday and will continue them in Denver on Dec. 2 and 3.

In its proposed rules, EPA distinguishes between surface water (such as the Potomac River, source of most of the D.C. metropolitan area's supply) and ground water, which is pumped from wells and other subterranean sources. Natural filtration of rainwater through layers of soil to the underground water table usually suffices to eliminate most of the parasites.

A study cited by EPA found evidence of Giardia in 40 to 50 percent of water samples from public supplies drawn from creeks, rivers and lakes, and 33 percent from springs but only 5 percent from wells.

Another study, which analyzed waterborne disease outbreaks from use of public water systems with surface water sources, ascribed 56.6 percent of the outbreaks and 58.6 percent of the individual sufferers to giardiasis. This probably is an underestimate because an additional 34 percent of outbreaks and 28.6 percent of victims were attributed to "gastroenteritis, undefined."

Of myriads of tiny organisms that cause disease, the Giardia "bug" has been known longer than most, said Dr. Theodore E. Nash, a senior scientist in the laboratory of parasitic diseases at the National Institute of Allergy and Infectious Diseases in Bethesda.

"Anton von Leeuwenhoek, one of the earliest users of a microscope back in the 17th century, identified Giardia in his own stool," Nash said. "But it was not commonly accepted as a cause of disease until some American travelers returned from Leningrad in the late '60s with diarrhea that was identified as being caused by this organism."

Though not known with certainty, it is assumed that these cases came from the former tsarist capital's public water supply.

Diarrhea in travelers is a hallmark symptom of giardiasis. Unlike many forms of gastrointestinal distress, this one is often hard to diagnose and harder to shake. What's more, symptomatic relief with over-the-counter remedies such as Pepto-Bismol does not usually help.

The Giardia microbe lives in the human gut, attaching onto the intestinal wall in true parasitic fashion, and multiplies by splitting in two, a process called fission. It is what Nash calls "a beautiful little thing" about one 2,000th of an inch long, thin and sinuous, with fringe-like flagellae along its sides.

Giardia cannot live outside its normal environment, so when it is expelled in feces, it encapsulates itself in a hard little cover called a cyst and goes dormant until once again it encounters a hospitable environment. These microscopic cysts are incredibly infective. "Ten cysts are infectious for 30 percent of people, and 100 cysts are infectious for 100 percent," Nash said.

Nash, who gets problem cases referred by other physicians, looks first for a domestic, rather than an exotic, source for the disease in most patients he sees.

"The patient usually has a young child, and often that child attends a day-care center. In some centers, up to 70 percent of kids are infected with Giardia," Nash said.

Even a meticulous mother, disinfecting her hands after changing diapers or washing baby clothes, can end up with enough cysts on her body to become infected unknowingly.

This apparently is what happened to Hilary Goulding, a 40-year-old Maryland mother of two boys, 5 1/2 and 2 1/2. The toddler had been in a summer program last July. By late August, Goulding was experiencing nausea, vomiting and diarrhea, and the child suffered milder intestinal distress. The mother's condition worsened, and she was admitted twice to George Washington Hospital for rehydration, according to Nash.

Goulding subsequently improved enough to go on a recuperative cruise in September. The diarrhea returned, and she was treated for dehydration on shipboard.

When she came to Nash, his examination established giardiasis as the disease, which responded to a drug called Atabrine (see box).

Tests of family members showed that the toddler "had a lot of Giardia," but the other son and her husband were not infected.

A similar case involved Gail Spell of Vienna, Va., a 52-year-old Fairfax County aerobic exercise instructor who became ill last August. Not long before, Spell had babysat two grandchildren, ages 3 and 2, who had diarrhea and "weren't feeling well."

Her first symptom, noted while on a trip to Ohio and Connecticut, was diarrhea, which was treated with antibiotics. This did not help, and soon Spell "couldn't eat or hold anything down, and had internal gas and a distended stomach."

Vomiting persisted over a three-week period. She also showed signs of malabsorption, or inability of the intestinal wall to take up nutrients for the body's use.

Spell eventually found adequate treatment at a medical center in Fairfax County and got a clean bill of health after two negative stool samples, three days apart, following a standard course of Atabrine therapy.

The Merck Manual, a common doctor's desktop reference, advises that "household and sexual contacts should be examined and if infected treated," but Nash believes this is overdoing things a bit if older members of the household are without symptoms.

When there is a child in the household, however, Nash agrees that he or she should be examined because of the potentially serious consequences from malabsorption. "An adult loses weight from malabsorption," Nash explained, "but in a child, growth can be stunted."

Many children with severe Giardia infection do not show symptoms, such as diarrhea, that might alert parents to possible trouble, Nash added.

No one is immune to giardiasis, but some groups are more likely than others to contract it. In addition to travelers and children in day-care centers, groups at high risk include homosexual men and persons in institutions for the mentally retarded, health officials say. Backpackers also are at higher-than-normal risk of giardiasis, since the disease is readily picked up in isolated areas favored by hikers.

Giardia cysts from animal feces are washed into watercourses by rainfall and snowmelt and, if not destroyed by water purification methods, can infect humans. No matter how pure an upland brook may appear, Nash warns, its water should be filtered or boiled -- not chemically treated -- before being swallowed. Boiling is best, he adds.

NIH's Nash believes, and EPA data support him, that just chlorination is not enough to rid water of Giardia cysts, though it cuts down significantly on the danger.

Filtration, a standard part of modern public water treatment in the D.C. metropolitan area as well as most other cities, sharply reduces risk. One survey cited by EPA showed that water that had been disinfected but not filtered was eight times as likely to let waterborne disease slip through than water that had been both disinfected and filtered.William Hines is a Washington writer.