India's variety of mangoes tempts travelers, but severe diarrhea could be the result of indulging without caution. Prashanth Vishwanathan for The New York Times

NEW DELHI - Accepting a just-picked mango from a stranger in Lodi Gardens and then putting it directly into my mouth - skin and all - was stupid. I admit that.

But why did my first horrible case of traveler's diarrhea in India have to result from a mango? I love mangoes.

"You didn't even wash it?" Dr. Paul Offit, chief of infectious diseases at Children's Hospital of Philadelphia, asked me.

No.

"Even by your standards, that was really stupid," Dr. Offit said.

But what about the local yogurt I had eaten and the probiotic pills I had taken - weren't my gastrointestinal flora protecting me?

As it turns out, the fight against toxic bacteria is largely waged by the body's immune system, not the sweet-tempered millions found in a spoonful of yogurt.

"An immune response is a much more powerful agent against these bacteria than is trying to rearrange things within your intestinal flora," Dr. Offit said.

That Indians are less likely than non-natives to be sickened by food-borne bacteria results less from their different intestinal flora than from years of experience, said Dr. David Relman, a microbiologist at Stanford University School of Medicine in California.

"Although it may appear that a lot of adults don't seem to develop diarrheal disease, they probably had a fair bit of it as kids, and it was through those episodes that they got immunized," Dr. Relman said.

But without experience fighting these new invaders, a visitor's immune system has little chance of preventing an illness. That is why quickly taking antibiotics is so crucial. They are often miraculous cures, because 80 percent to 90 percent of traveler's diarrhea cases are caused by bacteria, said Dr. Phyllis Kozarsky of the Centers for Disease Control and Prevention in Atlanta.

Imodium, also known as loperamide, is effective too. Again, this was news to me. Dr. Kozarsky said Imodium ameliorates the illness's highly disruptive symptoms - a significant benefit when the short trip to the bathroom seems like a marathon.

Still, there are risks associated with aggressive treatment of traveler's diarrhea. Antibiotic therapy can increase a patient's vulnerability to other toxic bacteria, Dr. Relman said. All the good bacteria normally inhabiting the human gut - bacteria that get decimated with antibiotics - somewhat inhibit toxic microbes from getting a foothold or flourishing, even if they cannot fight off a full-scale assault.

"If you take an antibiotic and then get on a plane to India, you're much more likely to develop a serious infection," Dr. Relman said.

Without those bacteria, the body may be more susceptible not only to other bacterial infections but even to viral ones like the flu, said Dr. Susan M. Huse of the Marine Biological Laboratory in Woods Hole, Massachusetts.

Some studies suggest gut bacteria could even play a role in the development of obesity.

To ward off diarrhea, Michael Fischbach, a microbiologist at the University of California, San Francisco, endorsed eating yogurt and other fermented foods, particularly after using antibiotics, but he said there is little evidence proving the effectiveness of this strategy.

Dr. Kozarsky recommended daily doses of Pepto-Bismol, which can reduce the risks of traveler's diarrhea during brief stays. Pepto-Bismol is not recommended for long periods, she said. Indeed, some antacid therapies can increase vulnerability to traveler's diarrhea.

She recommended that travelers limit meals to foods that resist bacteria or those that have been well cooked. "If you eat things that are still steaming, the bacteria will be killed," Dr. Kozarsky said.

Because I live in India now, I cannot abandon fresh fruits and vegetables. I soak them in diluted bleach - including the mangoes.

Because gut bacteria are now suspected by scientists of playing roles not only in keeping my weight down but also in protecting against a variety of chronic diseases, like autoimmune disorders and diabetes, I will continue to try to get through mild bouts of diarrhea without resorting to medication.

But if I develop a fever or really suffer, I plan immediately to take ciprofloxacin, a powerful antibiotic that is available over the counter in India.