When you get that bad gut feeling, Derek Cheng advises you to clench.
Your butt cheeks instantly tighten, your breath quickens and your eyes bulge in dread. Nothing turns a traveller pale with fear like the unmistakable growl of an upset tummy in the developing world.
That awful gut feeling dominates backpacker conversation throughout Asia, Africa, the Middle East and Latin America. "How are the bowels today? Solid and steady? A little loose?"
Most of the time you're near a toilet and things don't get too messy. But sometimes toilet-land is a faraway fantasy place, and no amount of cheek-clenching can prevent a disaster.
In Sucre, Bolivia, I succumbed to a foul-tasting coffee while walking in a park, right by a children's playground. As I was about to explode, I spotted a public toilet, ran through the front gate, ignoring a man demanding a usage fee, and reached the toilet just as the floodgates opened.
At least in Nepal I managed to keep my trousers clean. Walking in the Everest Valley, my guts suddenly seized. I didn't hesitate. I squatted on top of a 1m-tall stone wall, and promptly relieved myself on the other side of the wall. Luckily, the popular path was empty.
In Thailand, after a questionable curry, I was rock climbing - 30m above the ground - when my cheeks twitched. An emergency abseil ensued. I was still attached to the rope, my toes just grazing terra firma, when the inevitable was unleashed. Again, I was lucky to be alone.
Friends in Melaka, Malaysia took me to dinner at nearby street stalls. Driving home, I sat in the back, squeezing my cheeks together. In the driveway, I hopped from one foot to the other, politely waiting for the door to open when ...
I spent a few hours cleaning that driveway.
Travellers' diarrhoea usually stems from food or water that is contaminated by faeces. Direct contact is not necessary. The food may simply have been prepared by someone with dirty hands.
The resulting infection, often Escherichia coli, isn't always from your last meal. It can take days for the bacteria to line your intestine and release toxins. Often it's benign, with just a day or two of unpleasant and frequent toilet stops. Ninety per cent of cases resolve within a week.
There are various fixes. There's the extreme weight-loss method of toughing it out until your body rights itself. Imodium will plug you up - handy for overnight bus or plane journeys.
Antibiotics are effective, but the more often you take them, the more vulnerable you are to resistant infections.
Rehydration is always vital.
And, of course, there's prevention. Some foods - fruit and leafy greens, undercooked meats, tap water, unpasteurised dairy - are more risky. Street food is perilous. You could save your guts and eat only in nice hotels. Or McDonald's.
But food is the lifeblood of travel, and street food is especially memorable - local, cheap, delicious. Few sights are more intriguing than a stall offering something I've never seen before, with a rich aroma and haloed in culinary charm.
Satayed bull's heart in La Paz, Bolivia. Assam laksa in Penang, Malaysia. Nepalese momos (dumplings). I'd rather risk public humiliation than give up these potentially bowel-crippling delights.