Restaurant food tends to be riskier in general not only because more hands are involved in the preparation but also because the ingredients are ordered in bulk. “A fast-food hamburger could have meat from a hundred different cows,” said Dr. Fisher, and it takes only one with a pathogen to make you sick. “The eggs in your two-egg omelet were poured out of a carton so it could have come from 50 different chickens,” she said.

Similarly suspect are freshly made juices and smoothies which are extracted from pounds and pounds of produce. Just one speck of contaminated dirt in your detox drink could upend your gut. And think of all the hands that necessarily touched the produce from the time it was picked in the field to when it was chopped and crammed into the Vitamix.

Let’s not forget about germs on your own hands if you’re not diligent about washing them with soap and water (hand sanitizers don’t kill some of the bugs that make your stomach sick). Did you eat or otherwise put your fingers in your mouth after gripping the pole on the subway or after throwing a slobbery ball for your dog? Did you put your mobile phone down on the table at a coffee shop — or on top of the toilet-paper dispenser in a public restroom — and then put it up to your mouth to take a call? Germs that make it into your digestive tract don’t always come from food.

And sometimes your gut distress isn’t caused by a germ at all. It could be an overdose of fermentable oligosaccharides, disaccharides, monosaccharides and polyols, known in public health circles as Fodmaps. These are essentially carbohydrates that, eaten in excess, are not well absorbed in the small intestine and then make their way into your colon to cause all kinds of trouble. They include myriad things we’re encouraged to eat including broccoli, brussels sprouts, radicchio, asparagus, avocados, mushrooms, peaches, whole grains and legumes.

“People are trying to eat so healthy these days, but a lot of those things are high in Fodmaps,” said Dr. Scott Gabbard, a gastroenterologist at the Cleveland Clinic. “You could always eat a lot of salad, but on that certain day, that certain combination of fruits and vegetables in your salad was just high enough in Fodmaps that it overrode your system’s capability to absorb those carbohydrates and you ended up with something almost like a purging.”

Drugs are also a common source of acute gastrointestinal events. Dr. Fisher told the story of an otherwise healthy and active patient who had bouts of GI distress every few months or so. After many tests came back negative, she finally determined it was the ACE inhibitor the patient was taking to control his blood pressure.

“A side effect of the drug is that it causes swelling in the intestines so he would get these temporary, partial obstructions,” Dr. Fisher said. “He would vomit and skip a meal and then the swelling would go down. It went on and on until we took him off that drug and now he’s fine.”