Food, Sex and Salmonella. David Waltner-Toews

are switched on by both physical and chemical stimuli. As with diarrhea, the amount and timing of what comes out of your mouth could be measured, which is why I would call it a sign.

      But there is more to this story than what comes out the orifices of one’s body, instructive as that may be. Let us contemplate, for a moment, food as it passes our lips into the masticatory caverns where lurk the uncompromising teeth and the probing tongue. At the risk of self-absorbed intellectual implosion, let us further contemplate the tiny bacteria that may dwell within that food, whether they be friendly lactobacilli in yogurt, interloping Salmonella in cheese, or the parts per billion old or new chemicals that contaminate the interstices of complex carbohydrates or comprise the food itself. In so doing, you may gain some understanding of how you can get sick from so apparently a feeble sin as eating an egg soft boiled.

      Forget, for a moment, the candle, the wine, that ambiguous look across the table. This is what it comes to: food is ground up and mixed with saliva in the mouth. Saliva contains, among other things, amylase, an enzyme to aid in digesting starch. It serves, as well, as a solvent for food chemicals, thus allowing one to taste them. Taste is a first defense against foodborne disease. I am quite suspicious of so-called acquired tastes. Not all foods that can make you sick taste bad, but—certain Danish cheeses, Chinese eggs, and Scandinavian putrefied fish aside— foul tastes and odors, metallic tastes, and burning sensations should serve as a warning.

      Nevertheless, I confess that I, too, once polished off a bad-tasting steamed bun from a food court for a negligent nephew. It was from a Vietnamese fast food restaurant, and I felt a certain duty not only to eat the food but also to like it, as penance for coming from a culture that had behaved so miserably toward Vietnam. If I expected God to reward my virtue in finishing a bad-tasting bun, I was wrong. I not only felt miserable during the baked salmon candlelit dinner that evening but also spent what would have been the better part of my Christmas holidays bent over a porcelain throne, wishing I were dead. This kind of behavior may have something to do with parents who make an issue of food, drawing tight and judgmental arguments around the uneaten chicken and the starving children in India, China, or Ethiopia. Guilt and food are a common and deadly combination.

      Our mouths have a large and complex natural bacterial flora that rarely cause any disease, unless you bite someone. Moreover, bacterial infections may put you off your food, but they seldom attack your mouth directly. Occasionally, foodborne diseases are associated with a sore throat. Beta-hemolytic Streptococci, the cause of strep throat, have managed on at least two occasions in the last decade to use food handlers to perpetuate themselves: once through a chocolate mousse fed to people at a blood bankers’ conference in Kansas City and once through cold meatballs fed to soldiers at an Israeli military base.

      Toxins—either those inherent in some foods, those added by our bacterial friends, or those that we add deliberately or inadvertently—are another matter. In very high doses they may induce irritation at the back of the pharynx, and hence retching, which is different from vomiting. Some toxins may produce burning or tingling sensations. What they do at low doses is the topic of a later chapter.

      Safely past the teeth, the tongue, the tonsils, and the salivary glands, your food now takes that not-quite-irreversible plunge down the esophagus, white-water rafting on peristaltic waves down to your stomach. The image of a watery torrent is not so far off the mark. Consider for a moment the following estimated flow of fluids through your body every day: about 2 quarts of intake, 6 quarts of saliva and gastric, bile, and pancreatic juices, and 6 quarts of secretions from the intestines—for a total of 3 to 4 gallons! And these fluids are not just water; they contain all manner of essential electrolytes, such as sodium and potassium. Yet on a normal day in a normal person, less than a cup of that amount gets out the back door. Most of it is re-absorbed in the lower part of your intestines; this re-absorption is essential to maintain your body’s fluid and electrolyte balance. It does not take much imagination to consider the havoc that might be wreaked by a small hole in the dike, the work of a tampering bacterium or virus.

      At the stomach, the cardiac sphincter, the front door, opens to let food in but sometimes sprays a bit of acid up into the soft flesh of the esophagus, resulting in heartburn.

      The stomach accepts the now somewhat sorry-looking foods as hand-me-downs from the mouth, grinds them into smaller particles, and churns up an emulsion of the fats. Sloshing about in this very acidic fluid, the food undergoes some preliminary digestion. As one might imagine, for the bacteria stowing away in our food, this acid bath is the most potentially deadly part of the ride.

      If the collective bacterial mind thinks that our mouths provide easy access to the internal offices of the human body, they are sadly mistaken. The lumen, or inside, of the gastrointestinal tract, from mouth to anus, is technically outside our bodies. While the inside of the gut has direct connections with the world around you, and is full of all sorts of bacteria and viruses, the true “inside” of your body is sterile. A heart surgeon has to wear a mask and gloves to protect you from infection; my family physician checking my prostate gland wears a glove to protect her, not me. The gut is lined with protective barriers and guardian cells. It is their job to let in only those molecules that know the appropriate code words.

      A great many bacteria die in the stomach. How, one might ask, do any of them survive? Sometimes we unwittingly give them help. An antacid taken for the heartburn, for instance, helps neutralize the stomach’s acids and makes it more comfortable not just for you but for the little lives within you. Infants naturally have less acidic stomach contents and so are more likely to give the bacteria a home. Food may also protect the bacteria, which snuggle unmolested in its depths. Furthermore, some vessels give the bacteria a slower ride through the acid. Small amounts of carbohydrates may pass through the stomach in thirty minutes or less. A high-fat meal may take several hours to move on.

      With the exception of organisms such as Helicobacter pylori, which causes ulcers and is probably transmitted through contaminated food and water, bacteria rarely find the stomach sufficiently hospitable to take up residence there. Toxins are a different matter. They may irritate the stomach directly or may slip off to one’s neural vomit center, where they stimulate a relaxation of the cardiac sphincter and esophagus, with simultaneous contraction of the abdominal muscles and viscera. Throwing up is an apt description; one might imagine a small but powerful baseball player hiding in one’s depths.

      Toxins formed by the bacteria Staphylococcus aureus (which hang around many people’s noses) and Bacillus cereus (which prefer rice fields) may attack in this way. Metals such as copper, zinc, tin, and cadmium, if ingested in high doses, may do something similar. We might ingest such metals through leaching from a container or because our crops were grown in industrially contaminated soils.

      Now that your food has survived a trip through the stomach, your gastronomic happiness rests in the hands of the small intestine, a 23-foot-long glistening, slithery tube. Although many toxins that can damage the stomach also attack this part of the gut, it is here that infectious agents such as bacteria and viruses first enter the fray. To describe this place appropriately, one must stoop to the cryptic and bilious language of darkness and melodrama. Enzymes are secreted by the pancreas and by cells in the crypts lining the long, serpentine halls. Bile from the liver enters the first part of the small intestine, the duodenum, and makes the fat soluble. Food that is broken down is ingested through the absorptive cells clustered on the folds, called villi, which, for bacteria, is the plural of villa, a great place to luxuriate.

      Farther along, in the lower part of the ileum, most of the fluids and electrolytes mentioned earlier are absorbed. Few bacteria live naturally in the upper small intestine. Buffeted by peristaltic waves and rained upon with bile salts, they tend to move on in search of more hospitable environments. As well, the normal bacterial burghers of the lower small intestine fill up the available living space and effectively exclude, in most cases, all but the most aggressive newcomers. This phenomenon has been called competitive exclusion and is the rationale behind eating friendly bacteria, say, from yogurt, to prevent the pathogenic, disease-causing ones from moving into your neighborhood.

      The small intestine is the central starting place for most foodborne illnesses. Nausea and vomiting, as in the stomach, still occur but are less common the farther into the bowels the