Mexico is overly famous for the threat of bandidos and Moctezuma's revenge. The latter is a stereotypical buzz-word used to describe any stomach or intenstinal ailment experienced while on Mexican soil. The fact is that most tourists never contract diarrhea or nausea diseases while touring Mexico, but often experience an upset stomach from over dosing on too many greasy tacos, especially when followed by a couple dozen cervezas (beers).
The best way to avoid stomach problems in Mexico, is to take it easy for the first week, and indulge in fits of gluttony on a gradual scale. It's really hard to resist those totopos (chips) and salsa, sitting on the table, while waiting for your lunch to arrive. Go ahead and indulge yourself, but don't eat half a dozen bowlfulls of salsa and not expect gastrointestinal ramifications later in the day.
Fruits and vegetables should be washed in sterilized water (refer to the article on water elsewhere in the mini-series) so that a weak chlorine bleach solution kills offensive bacteria. Chicken should be treated like it is radioactive, until cooked (just as true in the U.S.). Wash your hands after handling untreated, raw food, and frequently when preparing food. I like to carry a small package of moistened "towelettes" when foraging through markets, and wash my hands every hour or so. Typical restaurants may have restrooms with cold running water, but no soap and no paper towels. I'll use a handy towellette instead.
Even if all of my careful precautions should fail, and I get sick anyway, I know that I am not helpless. Many well-meaning articles have suggested "toughing-it-out" with intestinal ailments, and "letting nature take her course". I think this isn't being realistic, especially if I happen to be in the middle of travelling to a different area. Instead, I checked with my doctor before I left home, and obtained a prescription for "Bacrtrim F" tablets (also available in Mexico without a prescription). Bactrim effectively annihilates offensive bacteria within twelve hours after dosing. To control watery stool, I also take Lomotil. I know instinctively, that letting things clear up on their own, is the "natural way". But while proponents are still doing the four-yard-dash inside their rigs, I'm buckled up and heading down the road. Occasionally an unfortunate forager will come into contact with food poisoning. The symptoms are usually quite pronounced and "miserable". Happily, the most common variety of food poisoning in Mexico (infrequently encountered however) lasts only 12 hours. The only cure for food poisoning is time. Sipping sugary soft drinks is a good way to maintain energy. At this point if you're clutching your stomach and muttering "no way!", then you've lost track of reality.
I've gone on long trips to Mexico, time after time, without so much as a sniffle, then the odds catch up, and I'm unpleasantly reminded to always talk about the subject accurately and subjectively. The truth is, that I've had the very same symtoms as Moctezuma's Revenge while residing in the United States (probably 100 times as often). The chances are about fifty to one, that you'll come into contact with the affliction named after the famous aztec emporer, on a two-month vacation. Rather than gnash your teeth over the subject, remember that Mexico has an excellent health care system, and that someone else is going to take your parking spot on that lovely beach if you are scared away by a tiny bacteria. As for myself, I worry more about avoiding a bad hangover, or missing out on a nearby fiesta, than I do about Moctezuma's Revenge.