Many different illnesses cause diarrhea, ranging from short-lived stomach bugs to chronic digestive conditions. The most common culprit is viral gastroenteritis, often called the “stomach flu,” but bacterial infections, parasites, food intolerances, medications, and autoimmune diseases can all trigger loose, watery stools. Diarrhea is generally defined as three or more loose stools per day, and the cause often depends on how long it lasts: acute diarrhea (a few days) is usually infectious, while diarrhea persisting longer than four weeks points to a chronic underlying condition.
Viral Infections
Viruses are the single most common cause of acute diarrhea. Two viruses account for the majority of cases. Norovirus is the leading cause of epidemic gastroenteritis in both children and adults, spreading through contaminated food, water, and surfaces. Symptoms include watery diarrhea, nausea, vomiting, stomach cramps, and low-grade fever. In adults, norovirus illness typically resolves in two to three days. Children under 11 often start with sudden vomiting, and their illness can stretch to four to six days.
Rotavirus is the principal cause of severe gastroenteritis in young children. It produces watery diarrhea lasting about five days, often preceded by sudden vomiting and accompanied by fever. Dehydration is a major concern, especially in infants and toddlers. Vaccination has dramatically reduced rotavirus hospitalizations, but it remains common in parts of the world with lower vaccine coverage. Other viruses that cause diarrhea include adenovirus (types 40 and 41), astrovirus, and sapovirus, all of which spread through the fecal-oral route.
Bacterial Infections
Bacterial diarrhea is often linked to contaminated food or water and tends to cause more intense symptoms than viral illness. Campylobacter is one of the most frequently identified bacterial causes. Symptoms, which include diarrhea (sometimes bloody), fever, and stomach cramps, typically appear two to five days after exposure and resolve within about a week. Undercooked poultry and unpasteurized milk are common sources.
Salmonella causes similar symptoms and is associated with raw or undercooked eggs, poultry, and produce. E. coli infections vary widely depending on the strain. Most cause mild, self-limiting diarrhea, but certain strains produce toxins that lead to bloody diarrhea and, in rare cases, kidney complications. Shigella, another bacterial cause, spreads easily in daycare settings and produces watery or bloody diarrhea with fever and cramping.
Parasitic Infections
Parasites cause diarrhea that tends to last longer than viral or bacterial illness, often persisting for weeks. Cryptosporidium is the leading cause of waterborne diarrheal disease in the United States. It produces prolonged, frequent, watery diarrhea along with stomach cramps, nausea, and weight loss. Symptoms begin about seven days after infection and usually clear within two to three weeks in healthy people, though they can come and go for up to 30 days. Recreational water, such as swimming pools and lakes, is a major transmission route because the parasite is resistant to chlorine.
Giardia is another waterborne parasite, commonly picked up from contaminated streams, lakes, or well water. It causes greasy, foul-smelling diarrhea, bloating, gas, and nausea that can last for weeks without treatment. Both parasites are diagnosed through stool testing and treated with specific prescription medications.
Food Intolerances
Not all diarrhea comes from infections. If you regularly get loose stools after eating certain foods, a food intolerance may be responsible. Lactose intolerance is the most common example. When your body lacks enough of the enzyme that breaks down the sugar in dairy products, that undigested sugar passes into your colon, where it draws water into the intestine (an osmotic effect) and gets fermented by gut bacteria. The result is diarrhea, bloating, gas, and cramping within a few hours of consuming milk, cheese, or ice cream.
Fructose malabsorption works through the same mechanism. When fructose from fruit juice, honey, or high-fructose corn syrup isn’t fully absorbed in the small intestine, it reaches the colon and triggers the same osmotic and fermentation effects. Identifying a food intolerance usually involves tracking symptoms alongside what you eat, sometimes followed by a breath test or elimination diet.
Chronic Digestive Conditions
When diarrhea lasts longer than four weeks, it is classified as chronic and often signals an underlying digestive disorder rather than an infection.
Irritable bowel syndrome (IBS) is one of the most common causes of chronic diarrhea. It produces stomach pain, bloating, and changes in bowel habits, but it does not cause inflammation or physical damage to the digestive tract. The diarrhea-predominant form of IBS tends to flare during periods of stress or after certain foods, and it is managed through dietary changes, stress reduction, and sometimes medication.
Inflammatory bowel disease (IBD), which includes Crohn’s disease and ulcerative colitis, is fundamentally different from IBS. IBD is an autoimmune condition that causes physical damage to the digestive tract, leading to chronic diarrhea (often bloody), abdominal pain, fatigue, weight loss, and sometimes symptoms outside the gut like joint pain or skin problems. It requires ongoing medical treatment to control inflammation and prevent complications.
Celiac disease is triggered by gluten, a protein found in wheat, barley, and rye. It causes damage to the lining of the small intestine, which interferes with nutrient absorption. Diarrhea, bloating, fatigue, and unintentional weight loss are typical symptoms. Both celiac disease and IBD can prevent your body from absorbing the nutrients it needs, but celiac disease is managed by strictly avoiding gluten, while IBD requires different treatment approaches.
Antibiotic and Medication Side Effects
Antibiotics are a well-known cause of diarrhea. They kill the harmful bacteria causing your infection, but they also destroy helpful bacteria in your gut. This disrupts the balance of your microbiome, reducing the number and variety of beneficial microbes. Most antibiotic-associated diarrhea is mild and stops once you finish your course of medication.
The more serious concern is a bacterium called C. diff, which can overgrow when antibiotics weaken the gut’s natural defenses. C. diff produces toxins that inflame the colon, causing frequent watery diarrhea, fever, loss of appetite, and abdominal pain. It is diagnosed through a stool test and requires specific treatment. Beyond antibiotics, other medications that commonly cause diarrhea include antacids containing magnesium, certain blood pressure medications, and some diabetes drugs.
Dehydration: The Main Risk
Regardless of the cause, the biggest immediate danger from diarrhea is dehydration. Your body loses water and essential electrolytes with every loose stool, and replacing them is critical. Signs of dehydration include extreme thirst, dark-colored urine, urinating less than usual, dizziness, lightheadedness, and fatigue. In infants and toddlers, watch for no wet diapers for three or more hours, no tears when crying, a sunken soft spot on the skull, or unusual drowsiness.
Oral rehydration solutions, which contain a balanced mix of glucose and electrolytes, are the most effective way to prevent and treat dehydration from diarrhea. The World Health Organization recommends them for all age groups and for all but the most severe cases. Plain water alone does not replace lost electrolytes, so drinks designed for rehydration are a better choice during active illness.
Signs That Need Prompt Attention
Most acute diarrhea resolves on its own within a couple of days. Certain symptoms, however, warrant a call to your doctor: diarrhea lasting more than two days in adults (or more than one day in children), a high fever, six or more loose stools per day, severe abdominal or rectal pain, stools that are black, tarry, or contain blood or pus, and any signs of dehydration. In infants, children born prematurely, or anyone with a weakened immune system, the threshold for seeking care is lower. If a child refuses to eat or drink for more than a few hours, or cannot keep down enough fluids to stay hydrated, that is a situation requiring prompt medical evaluation.