Could a person catch ulcers from a housefly?   As preposterous as that might seem, it could happen, at least indirectly.

About 90 percent of ulcers are now known to be caused by a slender, spiral–shaped bacterium, Helicobacter pylori, or H. pylori. The resilient organism, found in the stomachs of one fourth of American adults, is believed to spread from person to person by food or water contaminated by fecal matter.

Microbiologists at St. Elizabeth's Medical Center in Boston recently demonstrated that the common housefly can become a reservoir of H. pylori, carrying the organism on its skin and in its intestinal tract and passing the bacterium on in its excretions.

One does not have to be a scientist to envision a likely mode of transmission. Houseflies lay eggs, develop and feed on excrement — human or otherwise — then often land on unprotected food, where they might deposit the troublesome organism and pass it along to an unsuspecting consumer.

If that mode of transmission is correct, it would partly explain why H. pylori infection is most common in poor countries. Indoor plumbing, refrigeration and modern sanitation are often lacking, and H. pylori infection is nearly universal.

A Common Bug

How H. pylori is transmitted is only one of several unsolved mysteries about a bacterium that should become as familiar to people as E. coli, salmonella and pneumococcus. For one thing, scientists are still not sure exactly how H. pylori causes an ulcer, although they know how it is able to survive and flourish in the hostile acid environment of the stomach.

But the most pressing question is why, when so many people harbor the organism, often for their entire lives, do only a few develop ulcers? And why do even fewer develop two forms of stomach cancer that are also attributed to H. pylori infection? One strain of the organism has a gene that renders it more virulent than other strains. But is that the whole answer? Are human factors also involved?

There is no simple way to predict who is likely to get sick from an H. pylori infection, so people could be treated to eliminate the organism before illness strikes.

Does Stress Play a Role?

Although discovery of H. pylori has prompted experts to discount stress as a cause of ulcers, might stress–induced changes in body chemistry explain why some people are susceptible and others are resistant to ulcers caused by the bacterium?

Then there is the treatment dilemma: the inability so far to find a simple way to cure an H. pylori infection. Three or four drugs taken simultaneously are best able to knock out the organism, but the more complicated, costly and unpleasant the treatment regimen, the less likely people are to follow it and be cured.

Thanks to the discovery of H. pylori 15 years ago by Australian scientists Barry Marshall and J. Robin Warren, most people, who develop the symptoms of an ulcer can be spared the constant threat of recurrence once they are treated with appropriate antibiotics.

They no longer have to gobble antacids forever or live on a bland diet, avoiding all spices, alcohol and coffee to squelch the pain that arises in the upper abdomen whenever their stomachs are empty or they consume irritating foods. And although reducing stress is always a good idea, ulcer or no ulcer, relaxation therapy is not essential to keeping the distressing ailment at bay.

There are several ways to diagnose an H. pylori infection. The simplest and least expensive is a blood test that can detect antibodies to H. pylori. But the test cannot distinguish between a current active infection and a past one. A second more expensive blood test may be needed to be certain of the diagnosis.

A Breath Test

Amore costly but also more precise method is the recently approved urea breath test. The patient consumes a small amount of urea that has a special form of carbon. If H. pylori bacteria are present in the stomach, a short while later the patient will exhale carbon dioxide that has the special carbon.

With the tests doctors usually can avoid having to insert a fiber–optic tube into the stomach to examine the stomach lining and to take a tissue sample to test for H. pylori. Although it is the most accurate test, it is also the most expensive and discomforting, and it must be administered by a specialist in gastroenterology.

A test not yet approved relies on detecting the genes of the bacteria in dental plaque or stool samples. It is quick and painless, and it can identify the particular strain of the organism.

Experts recommend testing anyone who has a new or recurrent stomach or duodenal ulcer, including those who regularly take nonsteroidal anti–inflammatory drugs such as ibuprofen and aspirin, which are known produce ulcers in some people But people with chronic acid digestion need not be tested because treatment to eradicate H. pylori infection, if found, has not been shown to relieve the symptoms.

No Easy Treatment

Anyone with an active ulcer or a healed ulcer who is found to harbor H. pylori should be treated to eradicate the organism. But the bacterium does not go down without a good fight. No one drug is fully effective, and the organism readily develops resistance to antibiotics. Thus is important to hit it hard and effectively with the first round of treatment.

There are several treatment options, none of them easy or pleasant. All involve a two–week regimen that combines one or two antibiotics and a substance that coats stomach lesions or a substance that inhibits secretion of stomach acid or both. The cost of medication ranges from $30 to $200.

Two four–drug regimens that include the antibiotics metronidazole and tetracycline with bismuth ( Pepto–Bismol ), which coats the stomach, and omeprazole ( Prilosec ), an acid inhibitor, are the most effective, eradicating the organism in 95 percent of cases.

Although the drugs cost more than $100, and the treatment is complex, it pays to follow it closely if you want to be cured. Side effects, which may include diarrhea, taste disturbances, nausea and black stools, disappear when the treatment is completed.

—   Jane Brody writes about health and medicine for the New York Times.