Photo: Jann Lipka/This content is subject to copyright.

Should you one day travel to the overlooked desert nation of Djibouti, you will see from the window, as you land, what appears to be a large construction site adjacent to the airport. In fact, it’s a US military base, Camp Lemonnier: 3,500 people who live and work in retrofitted shipping containers, some stacked, some side by side, a Tetris of unadorned rectangular boxes. Other than the shrubs that grow in the drip from the air-conditioning units, there is no landscaping. Interior décor takes the form of emergency instruction placards (“Stop and listen to the Giant Voice …”) and framed chain-of-command portraits. In three days on base, I’ve seen a single item that one might class as luxury: one indulgent, cushy, costly item shipped here for no other reason than to add a little comfort to a soldier or sailor or airman’s life. Captain Mark Riddle requisitions Charmin Ultra Soft for the container that belongs to Naval Medical Research Unit 3. The sign on the door explains it: Diarrhea Clinical Trial.

The word alone makes people want to laugh: diarrhea. Riddle doesn’t fight this. On the contrary. He recruits study subjects through GOT DIARRHEA? signs on the backs of restroom stall doors. One of the photographs on the Stool Grading Visual Aid he created for participants in the current study comes from a Campbell’s Chunky soup ad. (“Look closely,” he’ll confide, “there’s a spoon sticking out.”) Nevertheless, for reasons you will come to understand, Riddle takes diarrhea very seriously. As he has put it, intending nothing funny, “I live and breathe this stuff.” I have heard him use the word sacred to describe a collection of frozen stool samples. Riddle would like military brass to take it seriously, too.

In past centuries, this took no convincing. Dysentery “has been more fatal to soldiers than powder and shot,” wrote William “Father of Modern Medicine” Osler in 1892. (“Dysentery” is an umbrella term for infections in which the pathogens invade the lining of the intestine, causing cells and capillaries to ooze their contents and creating dysentery’s hallmark symptom, the one that sounds like British profanity: bloody diarrhea.) For every American killed by battle injuries during the Mexican War of 1848, seven died of disease, mostly diarrheal. During the American Civil War, 95,000 soldiers died from diarrhea or dysentery. During the Vietnam War, hospital admissions for diarrheal diseases outnumbered those for malaria by nearly four to one.

Once germ theory gained acceptance and the mechanics of infection became known, microorganisms—and the filth they breed in, and the insects that deliver them—became targets of military campaigns. Suddenly there were Fly Control Units, sanitation officers, military entomologists. The US military has been involved in most of the major advances in preventing, treating, and understanding diarrheal disease. Cairo’s NAMRU-3, the parent unit of Mark Riddle’s humble container lab in Djibouti, has a four-star antidiarrheal pedigree. Its first director, Navy Captain Robert A. Phillips, figured out that adding glucose to rehydration fluids enhances intestinal absorption of salts and water. This meant rehydration could be achieved by drinking the fluids rather than making one’s way to a clinic to have them administered intravenously. This has been a lifesaver not only for people who fight in remote, medically underserved areas but for people who live there. A 1978 Lancet editorial called Phillips’s discovery “potentially the most important medical advance this century.”

The full name of Riddle’s study is Trial Evaluating Ambulatory Treatment of Travelers’ Diarrhea (TrEAT TD). “Travelers’ diarrhea” is another catch-all term. Most of it—at least 80 percent—is bacterial, with 5 to 10 percent viral (vomit typically joining the waterworks here) and a miscellaneous percentage from protozoa like amoeba or giardia. All of it is caused by contaminated food or water. There used to be a separate category called “military diarrhea” (military referring to the patients, not the explosive nature of their evacuations), but if you look at the responsible pathogens, the breakdown is almost the same.

Military diarrhea is travelers’ diarrhea, because service members are travelers—in places where you don’t want to be drinking the water. A survey conducted by Riddle, David Tribble, and others with the US Naval Medical Research Center revealed that from 2003 to 2004, 30 to 35 percent of military personnel in combat in Iraq experienced situations where they lacked access to safe food and water. In the early days of a conflict especially, combatants are like backwater backpackers, crapping in the dirt and waving the flies off whatever food the locals are peddling. In that same survey, 77 percent of combatants in Iraq and 54 percent in Afghanistan came down with diarrhea. Forty percent of the cases were serious enough that the person sought medical help.

For every person who shows up at morning sick call, four tough it out. Riddle would like to know why. The average bout of travelers’ diarrhea lasts three to five days. Why endure this, when some of the new antibiotics, Riddle’s data show, can have you back to normal in four to twelve hours? He’s been asking around, mostly at mealtimes. The tables in the hangar-size Dorie are arranged church basement–style, in long rows, so there’s always a friendly stranger across from you or at your elbow, someone new with whom to chat about loose bowel movements while you eat.

Riddle gets right into it this morning with the man to his left. The uniform identifies the man as a Marine sergeant, last name Robinson. “I’m in the Navy,” Riddle is saying, “and we’re looking at simplified treatment regimens for travelers’ diarrhea. We’re finding that a single dose of antibiotic and an anti-motility . . .”

Robinson looks up from his eggs. “Anti—?”

“Like Imodium,” I offer. “Stops you up.”

“Oh, absolutely not. You do not want to mess with Nature like that.” Robinson has the booming vocals and commanding bullnecked air of the actor Ving Rhames. One imagines Riddle going straight over to the lab after breakfast and tossing his data in the trash—What was I thinking?

“You have something bad in you, bad water or what have you? You got to pass it.” It’s like discussing diarrhea with the Giant Voice. “Defeat the purpose if you mess with that.”

We’ve been hearing this a lot. People think diarrhea is the body’s attempt to rid itself of invaders, or to flush out the toxins they produce. They won’t take an antimotility drug like Imodium because they think it interferes with the purge. But diarrhea is not something you are doing to pathogens; it is something they are doing to you. In varied and dastardly ways. Shigella and campylobacter, two common causes of bacterial dysentery, wield a toxin-delivering “secretion apparatus”—a hypodermic-cum-bayonet that injects toxins into cells in the intestinal lining, killing them and causing the fluid inside them to spill out. That spillage is part of the watery-stool scenario, but there’s more! With enough of those cells out of commission, the large bowel can no longer perform its duty as an absorber of water. Instead of food waste getting drier and more solid as it moves along the gastrointestinal tract, it stays liquid all the way along.

Sergeant Robinson has nothing more to say about diarrhea, but he would like Riddle to have a word with the people responsible for the packet of toilet paper in the combat field rations, or MREs (Meals, Ready-to-Eat). “It’s like this much.” He tears off a piece of napkin the size of a drink ticket. “To wipe your ass!” Riddle volunteers that Navy guys pack baby wipes. He may regret saying this, because Robinson counters that Marines just cut off a piece of their t-shirt. Which possibly sums up the whole Marine Corps–Navy relationship.

During his years at NAMRU-3 headquarters, in Cairo, Riddle regularly got hit with diarrheal infections, a result of “sampling the fecal veneer” at local eateries. Irritable bowel syndrome is a well-documented, little-publicized aftermath of diarrheal infections—especially severe or repeated bouts. If you talk to people who’ve recently been diagnosed with IBS, about a third of them will say that their symptoms began after a bad attack of food poisoning. Defense Department databases reveal a five-fold higher risk of IBS among men and women who suffered an acute diarrheal infection while deployed in the Middle East. Even the Veterans Administration recognizes IBS—as well as a form of arthritis called “reactive”—as one of the “post-infectious sequelae” of enteric infections. If patients can show that the condition developed following an infection with shigella, campylobacter, or salmonella during deployment, they’re entitled to benefits. Riddle estimates that the Defense Department could wind up spending as much money on these long-term consequences of food poisoning as it spends on post-traumatic stress disorder.

Why not prescribe antibiotics more widely? First, there’s the issue of antibiotic-resistant strains developing, though this is less of a concern with some of the newer regimens that wipe out infections in a single day—likely not enough time for a resistant strain to evolve and thrive. More worrisome, perhaps, is recent research showing that the colons of overseas travelers who treat their diarrhea with antibiotics, particularly in Southeast Asia, tend to become colonized with two species of “bad” bacteria that they then carry home and can spread around town. Both bugs may inhabit a traveler’s gut only briefly and cause no problems while they’re there, but they are dangerous to patients with weak immune systems. Here again, with the newer single-dose regimens, it may not be an issue.

These are largely first-world concerns. The week I returned from Djibouti, the World Health Organization released a statistic for annual deaths from diarrhea worldwide: 2.2 million. The estimate for ETEC alone is 380,000 to 500,000 deaths per year. Children especially are at risk because they dehydrate dangerously fast. The Centers for Disease Control and Prevention puts the daily toll for deaths from diarrhea in children under five at 2,195—more than from malaria, AIDS, and measles combined. (The Gates Foundation is funding the Navy’s efforts to develop an ETEC vaccine.)

Riddle traveled a lot in his twenties and recalls being hit by a realization. So much of people’s lives—their opportunities, their health and longevity—comes down to where they were born. “It’s so random,” he says. We’re over at his office, which is downstairs from his lab, in the same container. “It shouldn’t be that way. It shouldn’t matter where your parents happened to live.” He pauses for a jet ripping through a takeoff. At certain times of day, you get this every few minutes. It’s like having a desk under the tarmac at Heathrow. The commotion fades and Riddle resumes. “I went into medicine wanting to help the greatest number of people.” And then, just when I thought he’d gone all earnest on me: “I happened to fall into diarrhea.”

***

Yesterday I convinced the droll and adorable Camp Lemonnier public affairs officer, Lieutenant Seamus Nelson, to put a request in the daily email feed that goes out to everyone on base. (“… Mary is looking for individuals who would be willing to share a story about how a case of diarrhea has impacted them while engaged in operations… .”) Because really, how do you step into that conversation?

The interviews have been scheduled back-to-back, one man coming in as another leaves, the Public Affairs container having taken on the quiet, hangdog air of a Catholic confessional. We just listened to the commanding officer of an inshore boat unit that protects Navy ships from USS Cole–type terrorist attacks in the port of Djibouti City. He demonstrated the maneuvers using Seamus’s stapler as the “high-value asset” kept safe by a tape dispenser and a bottle of allergy pills, zigzagging across each other’s paths. An inopportune bathroom break would leave the stapler vulnerable to attack. Even if crew stick to their posts, their vigilance is compromised; “illness preoccupation” is an overlooked military liability of diarrhea.

We heard a similar tale from a bombardier. On a long sortie out of Diego Garcia island, the only crew member capable of operating the plane’s defensive equipment abruptly left his post to use the chemical toilet—while flying over Taliban-controlled Afghanistan. On the return flight, a faulty seal combined with the pressure differential between the toilet’s tiered chambers caused the contents to spew into the crew cabin. “Be assured,” he deadpanned, “this blue-brown precipitation affected the navigator’s ability to concentrate on his duties.”

Our 3:30 is retired from Special Operations, now working as a contractor. He was hit with diarrhea every time his team deployed. Because of this, he was never assigned any “long-range surveillance,” meaning counter-terrorism missions deep into insurgents’ turf. These missions, he says, entail hiding out in a hole, watching a particular spot—say, an intersection: who comes and goes, how many trucks drive through, at what time of day.

I ask whether he knows of a vital operation that might have been compromised because someone got a vicious case of food poisoning. He dismisses the very idea. “The guys they select for this type of work? They don’t have these types of problems. They’re selected for a reason.”

After he leaves, Seamus turns to us. “Wow, do you think that’s part of the screening for Special Operations? Give you some bad food, see how you do?” He’s joking, but in fact 20 percent of the population are what Riddle calls “nongetters”: people who can eat ceviche from street vendors, drink the water, never get sick. It would certainly be an asset. Riddle wonders whether Special Operators take antibiotics or Imodium prophylactically, just in case, before critical missions. Or are they just suffering in silence? The Camp Lemonnier Special Ops doctor—they have their own, natch—talked about the men’s reluctance to seek medical help lest they lose their Special Operator status.

Riddle and I have a lot of questions. Alas, no one from Special Operations replied to the diarrhea email.

***

Seamus Nelson is six foot three. When he extends his neck to its full reach, his head is like a periscope. It’s up now, surveying a sea of clean-shaven, supper-chewing heads in the Camp Lemonnier dining facility. He’s scanning for facial hair. Only two categories of men here are allowed to wear beards: Special Ops and civilian contractors who want to look like Special Ops.

“There’s your guy.” The neck now retracted. “Far corner by the door.”

Riddle and I rise from our seats. We saw this man yesterday, coming out of the tactical shop. Even without the beard, you’d know he’s one of them. There are men who attempt to broadcast toughness by what they wear or drive or have tattooed on themselves. And there are others, like this man, who do nothing to cultivate or consciously project it, and yet it is obvious. It accretes naturally of the things they’ve experienced.

Besides, I saw him go into the secure zone.

“Seamus, come with me. Introduce me.”

We cross the cafeteria, nervous middle-schoolers at the dance. The man sees us but does not alter his expression. We stop a couple feet back from the table. Some kind of attitudinal concertina wire. Seamus plunges ahead. “Mind if we join you for a second?”

I’m going to assume the man is Special Operations, and that he knows we know. “I was wondering whether you might ever have been in a situation where … in a critical mission that …” I back up. “Well, because diarrhea is looked on as sort of a silly—”

“It’s not.”

He speaks softly, and what he says next I can’t quite make out. Something about being curled up in a hole in the fetal position. He says that where he just got back from, some unnamed “out station” in Somalia, it hits everyone. This is probably not exaggeration. In Riddle’s survey of diarrhea in Iraq and Afghanistan, 32 percent of respondents reported having been in a situation where they couldn’t get to a toilet in time. And Special Operators in the field get sick twice as frequently as everyone else.

His name, he says, is Carey. He invites us to sit down. I place my tape recorder in plain view—that is to say, in plain view of anyone on my side of the table. That is also to say, behind the condiment caddy.

I need Carey to set the scene. “What if you … I mean, what if someone were a sniper, and they’re in a hide for … well, how many hours would it be?”

“Depends on the mission. You’re watching for something to happen that might not happen.”

“Right, and most likely you’re out in some village, and you’ve had to be eating stuff that’s not prepared as hygienically as—”

“Goat,” he says. I had heard a story earlier about a goat meal in rural Afghanistan. It contained the phrases “singed hair” and “otherwise uncooked.” Unsanitary conditions, Carey confirms, are a given. “Unfortunately, we don’t fight in first-world countries.”

Carey says he does not, as Mark Riddle had heard some men did, take antibiotics or Imodium prophylactically before the mission or after the goat. He takes one precaution. It is a strict rule among Special Operators. “You go to the bathroom before going into a danger situation.” There has been no shift from the gravely quiet tone with which Carey has been speaking. Nonetheless, Seamus blurts, “Kind of like a road trip with the family, and Dad’s like, ‘I don’t care that you don’t need to go.’”

On a family road trip, no one has you in the sights of a semiautomatic rifle while you squat in the dirt. Historian of military medicine A. J. Bollet quotes a letter written by a Civil War soldier who explained that an unwritten code of honor forbade the shooting of a man “attending to the imperative calls of nature.” In the war on terror, there’s no such etiquette.

Excerpted from Grunt: The Curious Science of Humans at War, by Mary Roach. Copyright © 2016 by Mary Roach. With permission of the publisher, W.W. Norton and Company, Inc. All rights reserved.