People get diarrhea when something disrupts the normal balance of fluid in the intestines. Your gut processes roughly 9 liters of fluid every day, absorbing most of it back into the body. When infections, food reactions, medications, or chronic conditions interfere with that absorption, or when the intestines start actively pumping extra fluid out, the result is loose, watery stool. The causes range from a norovirus picked up at a restaurant to a glass of milk your body can’t digest.
How the Gut Loses Control of Fluid
Not all diarrhea works the same way inside your body. There are three distinct mechanisms, and knowing which one is at play helps explain why symptoms vary so much from one episode to the next.
The first is osmotic diarrhea. This happens when something you swallowed can’t be absorbed properly, so it sits in the intestine and pulls water in by osmotic force. Think of it like a sponge that draws fluid toward itself. Lactose in someone who is lactose intolerant is a classic example: without enough of the enzyme to break it down, the sugar passes into the colon intact, bacteria ferment it into short-chain acids, and those acids raise the osmotic pressure so much that the colon can’t reabsorb water fast enough. Sugar alcohols like sorbitol (found in sugar-free gum and candy) work the same way. The intestine absorbs sorbitol poorly, so it accumulates in the colon, traps water, and triggers loose stools. This type of diarrhea typically stops when you stop eating the offending food.
The second is secretory diarrhea. Here, the intestinal lining is actively pumping chloride and water into the gut. Cholera is the textbook case: the toxin hijacks a signaling molecule inside intestinal cells, which simultaneously stimulates fluid secretion and blocks sodium absorption. The two effects reinforce each other, and stool volumes can exceed a liter per hour in severe cases. Unlike osmotic diarrhea, secretory diarrhea persists even if you stop eating entirely, because the problem isn’t what you consumed. It’s the intestinal cells themselves misfiring.
The third is inflammatory (sometimes called exudative) diarrhea. When the intestinal lining is damaged, whether by an invading pathogen like Shigella or a condition like ulcerative colitis, the barrier breaks down. Fluid, mucus, proteins, and sometimes blood leak through the compromised tissue into the gut. This type often comes with fever, cramping, and visible blood or mucus in the stool.
Infections: The Most Common Trigger
Infections cause the majority of acute diarrhea episodes worldwide. The pathogens responsible split into three categories: viruses, bacteria, and parasites. Nearly all of them spread the same way, through food or water contaminated with fecal matter, or through person-to-person contact when hygiene is poor.
Viruses are the leading culprits. Norovirus is the most common cause of foodborne diarrhea in adults. It spreads easily in close quarters like cruise ships, dormitories, and restaurants, and takes very few viral particles to cause infection. In young children, rotavirus historically dominated, though widespread vaccination has reduced its impact significantly. Adenovirus and astrovirus also cause diarrheal illness in children.
Bacterial infections tend to cause more severe symptoms. E. coli, Salmonella, Shigella, and Campylobacter are the most frequent bacterial causes. Some strains of E. coli produce toxins that trigger secretory diarrhea, while Shigella and certain other bacteria invade the intestinal wall and cause inflammatory diarrhea with bloody stool. These bacteria commonly enter the body through undercooked meat, unwashed produce, or contaminated water.
Parasites like Giardia, Cryptosporidium, and Entamoeba tend to cause longer-lasting illness. They’re often picked up from untreated water sources, whether while traveling, camping, or in areas with poor sanitation infrastructure. Parasitic diarrhea can persist for weeks if untreated.
How Food and Drink Cause Diarrhea Without Infection
You don’t need a pathogen to get diarrhea. Certain foods and ingredients directly trigger the osmotic mechanism described above. Lactose intolerance affects a large portion of the global population, and even people who tolerated dairy fine in childhood can lose the ability to digest it as adults. The undigested lactose ferments in the colon, producing gas, bloating, and diarrhea.
Sugar alcohols are another frequent offender. Sorbitol, mannitol, xylitol, and erythritol appear in sugar-free candies, chewing gum, protein bars, and diet beverages. Because the small intestine absorbs them poorly, they accumulate in the colon, raise osmotic pressure, and prevent water from being reabsorbed. Some people notice symptoms after just a few pieces of sugar-free gum. High-fructose foods can have a similar effect in people who absorb fructose inefficiently.
Caffeine and alcohol both speed up intestinal contractions, which means food and fluid move through too quickly for the colon to absorb enough water. Large amounts of either can cause loose stool even in people with no underlying digestive issues. Spicy foods irritate the gut lining in some individuals, producing a mild inflammatory response that accelerates transit.
Antibiotics and the Gut Microbiome
Antibiotics are one of the most common medication-related causes of diarrhea. They work by killing bacteria, but they don’t distinguish between harmful pathogens and the beneficial species that live in your colon. When those protective bacteria are wiped out, the gut loses its ability to properly ferment fiber, absorb fluid, and resist colonization by harmful organisms.
A large Swedish study of nearly 15,000 adults found that a single course of certain antibiotics reduced the number of bacterial species in the gut for years afterward. One course of clindamycin was associated with an average of 47 fewer species detected in the gut a year later. Fluoroquinolones and flucloxacillin were associated with 20 and 21 fewer species, respectively. Gut diversity recovered most quickly in the first two years but remained significantly reduced even four to eight years after treatment.
The most dangerous consequence of antibiotic-related microbiome disruption is C. difficile infection. This bacterium thrives when competing gut bacteria have been eliminated. It produces toxins that cause severe inflammatory diarrhea, and infections can become life-threatening, particularly in older adults or hospitalized patients. C. difficile diarrhea typically begins during or shortly after a course of antibiotics, though it can appear weeks later.
Chronic Conditions That Cause Ongoing Diarrhea
When diarrhea lasts four weeks or longer, the cause is usually something other than a simple infection. Two of the most common chronic culprits are irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), and despite similar-sounding names, they work very differently.
IBS with diarrhea (IBS-D) involves no visible damage to the intestinal lining. The gut looks structurally normal, but the nerves and muscles controlling digestion are hypersensitive. Stress, certain foods, and hormonal shifts can trigger episodes of urgent, watery diarrhea that alternate with periods of normal bowel function or even constipation. The exact mechanism isn’t fully understood, but it involves abnormal communication between the brain and the gut.
IBD, which includes Crohn’s disease and ulcerative colitis, involves actual inflammation and damage to the intestinal wall. This is inflammatory diarrhea at its core. The immune system attacks the gut lining, causing ulcers, bleeding, and fluid leakage into the intestine. Symptoms often include bloody stool, weight loss, fatigue, and fever. IBD is a progressive disease that requires ongoing medical management.
Celiac disease is another chronic cause. In people with this autoimmune condition, eating gluten triggers an immune response that damages the small intestine’s absorptive surface. The result is a combination of osmotic and secretory diarrhea: nutrients aren’t absorbed (creating an osmotic pull), and the damaged lining actively secretes extra fluid.
How Handwashing and Sanitation Reduce Risk
Because most infectious diarrhea spreads through the fecal-oral route, the single most effective prevention strategy is handwashing. Pooled data from community trials involving nearly 15,000 people found that handwashing promotion reduced diarrhea incidence by 28%. In school and daycare settings, where children are in close contact, the reduction was 31% across trials involving more than 50,000 children. Programs that provided soap showed a stronger effect (34% reduction) than those that simply promoted handwashing without supplying soap.
Beyond hand hygiene, safe water and proper sanitation infrastructure are critical. Water contaminated with human or animal feces is the primary vehicle for diarrheal pathogens in low-resource settings. Proper food handling matters everywhere: cooking meat to safe temperatures, washing produce, refrigerating leftovers promptly, and avoiding cross-contamination between raw and cooked foods all reduce the chance of bacterial and parasitic infection.
For diet-related diarrhea, prevention is more personal. If you know you’re lactose intolerant, limiting dairy or using lactase supplements before eating it can prevent symptoms. Checking labels for sugar alcohols, moderating caffeine and alcohol intake, and introducing high-fiber foods gradually rather than all at once can help keep your gut’s fluid balance on track.