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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, and India’s vast array of deliciously different mango varieties has been one of the great delights of moving here.

“You didn’t even wash it?” Dr. Paul Offit, chief of infectious diseases at Children’s Hospital of Philadelphia, asked me later.

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? Since we all carry 10 times as many bacterial cells as human ones, wasn’t I for all intents and purposes already more Indian than American?

“Yogurt probably won’t hurt you, unless it’s contaminated as well,” Dr. Phyllis Kozarsky, an expert on traveler’s health at the Centers for Disease Control and Prevention, said in an interview. But there is no food on the planet that will protect against an onslaught of toxic bacteria, she added.

Despite decades of immunological research and a recent surge of interest in the bacterial garden of the human gut, diarrhea remains the most unpredictable travel-related illness. There is a grim acceptance among Western expatriates and visitors here that they will be felled by it — often on multiple occasions.

And there is a host of myths surrounding traveler’s diarrhea, many of which I have cheerfully perpetuated to family and friends. (Well, mostly to my wife.) There are also intriguing mysteries about how natives gain immunity to the food- and waterborne bacteria that prove so toxic to non-natives.

I have lived in India for four months, and I have been in gastrointestinal distress five times — roughly once a month. Part of the problem is that Indians are a very hospitable people. Almost everywhere I go, someone offers me food and drink, forcing me to quickly weigh the chance of contamination against the likelihood that a refusal would cause offense.

Beyond this coldhearted calculus is my very real desire to connect with people. The mango in question was given to me by a woman with such obvious good will that I found refusing her difficult.

I bit into the mango, putting my mouth on the peel to suck out the juice and pulp. As I did this, my brain engaged. A bird might have covered the mango in droppings. The mango might have fallen on the ground. The woman handing it to me might have been covered in toxic E. coli (more than 600 million people in India have no routine access to bathrooms).

Within hours, my stomach was in an uproar and I was seized with a fever. I knew I was in trouble because my first three cases of traveler’s diarrhea had been mild enough that I managed to get through them without taking any medicines, something I considered a personal triumph mostly because it allowed me to compare myself favorably with my wife.

Indeed, my wife joined me for the first week of my stay here before returning temporarily to the United States, and within four days she became terribly ill. I freely dispensed what turned out to be terrible advice, suggesting in the early hours of her illness that she avoid taking one of the antibiotic pills that we had brought for just such an eventuality.

My advice sprang from the mistaken belief that the good bacteria in her gut had a fighting chance against the bad bacteria. “Honey, taking an antibiotic is like carpet-bombing a battlefield,” I told her in confident tones. “You kill off all the good guys as well as the bad guys. Let’s see if the good guys rally first.”

They did not. 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. The wait, of course, simply ensured that the toxic bacteria were allowed to thrive and make her very sick, delaying her return home.

Oops.

That Indians are less likely to be sickened by the foods that felled my wife and me results less from their different intestinal flora than from years of hard-won immunologic experience, said Dr. David Relman, a microbiologist at Stanford University School of Medicine.

“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, my wife’s immune system had little chance of preventing her 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, Dr. Kozarsky said.

Imodium, also known as loperamide, is effective too. Again, this was news to me. Initially, my wife decided against taking Imodium because she thought doing so would lengthen the duration of illness. I concurred. Wrong again.

There is no evidence that Imodium lengthens the illness, Dr. Kozarsky said. Instead, it simply 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 that normally inhabit 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, Mass.

Studies also have suggested that the obsessive cleanliness in the West and overuse of antibiotics may be creating a generation whose immune systems, perhaps too protected from attack in childhood, are now constantly on overdrive, resulting in far more asthma and allergies. My 7-year-old son has had both problems, and while I am hopeful that the seemingly plentiful array of different bacteria here may help him, I certainly do not wish for him to get horribly sick in the meantime.

Indeed, repeated cases of traveler’s diarrhea can damage the gut’s ability to absorb nutrients and cause stunted growth in children, Dr. Relman said. Some studies suggest gut bacteria could even play a role in the development of obesity.

If there is a benefit from exposure to bad bacteria, it most likely occurs in infancy, when immune systems are still developing, said Michael Fischbach, a microbiologist at the University of California, San Francisco. My son is probably too old for new bacterial exposures to retrain his immune responses.

To ward off diarrhea, Dr. Fischbach endorsed eating yogurt and other fermented foods, particularly after using antibiotics, although 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 contracting 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.

Since we live here, we cannot abandon fresh fruits and vegetables. Instead, we soak them in diluted bleach — including our 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.

My repeated bouts of serious illness meant that I was no more robust or prepared for India than my wife, a tough blow to my ego. I have been sick more days than she. Living here will make me less vulnerable over time, but I no longer strive to become as resistant to infections as my Indian neighbors.

To achieve that, “you would suffer a variety of illnesses,” Dr. Offit said. “And under the assumption that suffering is bad, I wouldn’t recommend it.”

Gardiner Harris is a correspondent in New Delhi for The New York Times.