Viral infections cause most cases of diarrhea in children, but the list of possible triggers runs from antibiotics to fruit juice to food allergies. Understanding the likely cause helps you figure out whether the episode will resolve on its own in a day or two, or whether something else is going on that needs attention.
Viral Infections: The Most Common Cause
Viruses account for the vast majority of acute diarrhea episodes in kids. The usual suspects are rotavirus, norovirus, adenovirus, and astrovirus. These spread easily through contaminated hands, surfaces, and sometimes food or water, which is why outbreaks tear through daycares and schools so quickly.
Before the rotavirus vaccine became routine in 2006, rotavirus alone caused roughly 3.5 million cases per year in the United States and was the single leading cause of infant diarrhea worldwide. That picture has shifted dramatically in countries with high vaccination rates. The vaccine prevents an estimated 74% of rotavirus deaths and cuts related hospitalizations by roughly half. In the U.S., norovirus has now taken over as the top viral culprit, responsible for 19 to 21 million diarrheal illnesses annually across all ages and about 90% of gastroenteritis outbreaks worldwide.
Adenovirus, sapovirus, and astrovirus together account for another 2% to 9% of viral gastroenteritis cases globally, and they tend to hit children harder than adults. Most viral diarrhea comes with vomiting, low fever, and stomach cramps, and resolves within a few days without specific treatment. The main risk is dehydration, not the infection itself.
Bacterial and Parasitic Infections
When diarrhea is bloody, lasts longer than a few days, or comes with high fever, a bacterial infection is more likely than a virus. Salmonella, certain strains of E. coli, Campylobacter, and Shigella are the most common bacterial causes in children. These typically come from undercooked meat, contaminated produce, or contact with animals. Symptoms tend to be more intense than viral illness, with cramping, mucus or blood in the stool, and fevers above 101°F.
Parasites are a less common but important cause, especially in daycare settings. Giardia is the classic example. It spreads through contaminated water, unwashed food, or person-to-person contact during diaper changes. Symptoms typically appear one to three weeks after exposure, which makes it harder to trace the source. Giardia causes watery, foul-smelling diarrhea that can persist for weeks if untreated, along with bloating and gas. Child care workers and parents changing diapers are at higher risk, and outbreaks in daycare centers happen often.
Antibiotics
About 11% of children develop diarrhea while taking oral antibiotics. It typically starts around five days into the course and lasts an average of four days. Antibiotics don’t just kill the bacteria causing an infection. They also wipe out helpful bacteria in the gut, which disrupts normal digestion and allows certain organisms to overgrow.
The risk isn’t equal across all antibiotics. One common combination used for ear infections and sinus infections (amoxicillin-clavulanate) causes diarrhea in roughly 23% of children who take it. Kids under age 2 are especially susceptible. This type of diarrhea is usually watery, not bloody, and resolves after the antibiotic course finishes. If your child develops severe or bloody diarrhea during antibiotic treatment, that warrants a call to your pediatrician, since it can occasionally signal a more serious gut infection.
Too Much Fruit Juice
Excessive fruit juice is one of the most overlooked causes of chronic, loose stools in toddlers. Apple juice is a particularly common offender. The problem is fructose: children’s small intestines often can’t absorb all the fructose in a large serving of juice. The unabsorbed sugar draws water into the intestine and gets fermented by gut bacteria, producing gas, bloating, and watery diarrhea.
One study found that 65% of children malabsorbed carbohydrates from apple juice, compared to 33% of adults. Fructose, rather than sorbitol (which is often blamed), appears to be the primary sugar driving this effect. This condition sometimes gets called “toddler’s diarrhea” or chronic nonspecific diarrhea. The child seems perfectly healthy otherwise, is growing normally, and has no fever. The fix is straightforward: cut back on juice. The American Academy of Pediatrics recommends no juice at all for children under 1, and no more than 4 ounces per day for toddlers ages 1 to 3.
Food Allergies and Intolerances
Lactose intolerance can develop temporarily after a bout of viral gastroenteritis. The virus damages the lining of the small intestine, which temporarily reduces the body’s ability to break down lactose (the sugar in milk and dairy). This means a child who normally tolerates milk just fine may develop diarrhea, gas, and bloating for days or weeks after a stomach bug. Dairy can usually be reintroduced gradually over a week or longer once the gut has healed.
A less well-known but important cause is food protein-induced enterocolitis syndrome, or FPIES. This is a type of food allergy that doesn’t cause the classic hives-and-swelling reaction. Instead, it targets the gut directly. In the acute form, a child develops repetitive vomiting one to four hours after eating a trigger food, sometimes followed by watery or bloody diarrhea within 5 to 10 hours. Severe episodes can cause lethargy, pallor, and dehydration. In the chronic form, which is more common in infants under 6 months, daily exposure to a trigger (often cow’s milk or soy formula) causes ongoing diarrhea, vomiting, and poor weight gain. Symptoms resolve within days to weeks once the trigger food is removed.
Celiac Disease
Celiac disease is an autoimmune condition triggered by gluten, a protein found in wheat, barley, and rye. In children, it commonly causes bloating, abdominal pain, foul-smelling stools, and chronic diarrhea, though constipation can also occur. What makes celiac disease particularly concerning in kids is its effect on growth. Because the immune reaction damages the lining of the small intestine, nutrient absorption suffers. Children with undiagnosed celiac disease may develop iron and vitamin D deficiencies, slowed growth, short stature, and delayed puberty.
Celiac disease affects roughly 1 in 100 people and often runs in families. If your child has persistent digestive symptoms along with unexplained slowing on their growth chart, celiac disease is worth discussing with their doctor. A blood test can screen for it, and a biopsy of the small intestine confirms the diagnosis.
Recognizing Dehydration
Regardless of the cause, the biggest immediate concern with diarrhea in children is fluid loss. Kids dehydrate faster than adults because of their smaller body size and higher metabolic rate. Knowing the signs helps you gauge when to worry.
Mild dehydration (around 3% to 5% of body weight lost) may only show up as decreased urine output, with fewer wet diapers than usual. At moderate dehydration (6% to 10%), you’ll notice a dry mouth, skin that doesn’t bounce back quickly when pinched, a faster heart rate, and increased irritability. Severe dehydration (more than 10%) is a medical emergency: the child may appear extremely ill, with lethargy, altered consciousness, rapid breathing, low blood pressure, and mottled skin.
For mild to moderate cases, oral rehydration solutions are the first line of defense. These contain the right balance of salts and sugars to help the body absorb fluid efficiently. Plain water, soda, and sports drinks don’t have the right electrolyte balance for young children. In many parts of the world, the WHO and UNICEF also recommend zinc supplementation (20 mg per day for 10 to 14 days) alongside rehydration for acute diarrhea in children, as it has been shown to reduce the duration and severity of episodes.