Baby diarrhea is most often caused by a viral stomach bug, but it can also result from food sensitivities, antibiotics, or a change in diet. Before anything else, it helps to know whether your baby actually has diarrhea, because normal infant stool, especially in breastfed babies, is surprisingly loose and frequent.
Normal Stools vs. Actual Diarrhea
Breastfed babies naturally have soft, runny, sometimes seedy stools that are yellow or green. They can easily pass more than six stools per day, and those stools may even have a watery ring around them on the diaper. All of this is normal. Formula-fed babies tend to have thicker, peanut-butter-consistency stools, usually one to four times a day after the first couple of weeks.
True diarrhea means a sudden increase in both frequency and wateriness compared to your baby’s usual pattern. A good rule of thumb: if your baby passes three or more unusually watery stools, that counts as diarrhea. Mucus or blood in the stool, foul smell, poor feeding, fever, or generally acting sick all point toward something more than normal variation.
Viral Infections: The Most Common Cause
In most cases, a stomach virus is behind your baby’s diarrhea. Rotavirus and norovirus are the two biggest culprits. Rotavirus symptoms typically show up about two days after exposure and include severe watery diarrhea, vomiting, fever, and stomach pain. The vomiting and diarrhea usually last 3 to 8 days before resolving on their own. Norovirus follows a similar pattern but tends to be shorter, often clearing within one to three days.
These viruses spread incredibly easily through contaminated hands, surfaces, and objects. In group childcare settings, outbreaks are common. There is a rotavirus vaccine given in the first months of life that significantly reduces the severity of infection, though vaccinated babies can still get milder cases.
Cow’s Milk Protein Allergy
If your baby’s diarrhea is persistent rather than sudden, a sensitivity to cow’s milk protein is worth considering. Somewhere between 2% and 5% of young children have this allergy, and it can affect both formula-fed and breastfed infants (since milk proteins from a mother’s diet pass into breast milk, though the rate in exclusively breastfed babies is much lower, around 0.4% to 0.5%).
The symptoms depend on the type of reaction. Some babies develop bloody streaks or mucus in their stool as early as 2 to 8 weeks of life. Others show chronic diarrhea along with poor weight gain, starting around 2 to 7 months. Skin rashes, eczema, excessive fussiness, and vomiting can also appear alongside the digestive symptoms. If you notice a pattern of loose stools combined with any of these signs, your baby’s doctor can guide you through an elimination trial to confirm the diagnosis.
Antibiotics and Gut Disruption
If your baby recently started antibiotics, that is a very likely explanation. Diarrhea occurs in more than one third of patients who take antibiotics. The medication kills off beneficial gut bacteria along with the harmful ones, disrupting the balance of the digestive system. This can cause looser, more frequent stools that typically improve once the antibiotic course ends. Don’t stop the antibiotic without talking to your baby’s doctor, but do mention the diarrhea so they can assess whether a change is needed.
Bacterial and Parasitic Infections
Less commonly, diarrhea can be caused by bacteria like Salmonella or Campylobacter, or by parasites like Giardia. These tend to produce more dramatic symptoms. Bacterial infections often cause bloody or mucus-filled stools with high fever. Giardia produces distinctively greasy, foul-smelling stools that may float. It spreads through contaminated water, food, or surfaces.
These infections are less common than viral causes but more likely to need specific treatment. If your baby’s stool contains blood, looks greasy and unusually foul, or if diarrhea lasts longer than a week, a stool test can identify the specific organism.
Does Teething Cause Diarrhea?
This is one of the most persistent beliefs in parenting, but the medical consensus is clear: teething does not cause diarrhea. The timing is coincidental. Babies begin teething around 4 to 7 months, which is the same age they start putting everything in their mouths, increasing their exposure to germs. They’re also losing some of the passive immunity they received at birth. The real risk of blaming diarrhea on teething is that parents may overlook dehydration or an actual infection that needs attention.
Spotting Dehydration Early
Dehydration is the main danger of diarrhea in babies, and it can develop quickly because infants have such small fluid reserves. Watch for these signs:
- Fewer wet diapers than usual (fewer than six in 24 hours for a young infant is concerning)
- A sunken soft spot (fontanelle) on top of the head
- Dry mouth and lips
- No tears when crying
- Unusual sleepiness or irritability
Mild dehydration can be managed at home with extra fluids. For breastfed babies, nurse more frequently. Breast milk is an ideal rehydration fluid, and you should not replace feedings with water or other liquids. For formula-fed babies or older infants, an oral rehydration solution (sold under brand names like Pedialyte) replaces both fluids and electrolytes more effectively than water, juice, or sports drinks. Offer small sips every 10 minutes, gradually increasing the amount as your baby tolerates it.
When Diarrhea Needs Urgent Care
Most infant diarrhea resolves on its own within a few days. But certain signs warrant immediate medical attention:
- Your baby is younger than 6 months (smaller babies dehydrate faster and have less margin for error)
- Fever of 104°F (40°C) or higher
- Blood in vomit or stool
- Vomit that is green or bile-colored
- Inconsolable crying or extreme irritability
- Your baby seems unusually limp, unresponsive, or difficult to wake
- Signs of significant dehydration (sunken fontanelle, very few wet diapers, no tears)
Feeding During Diarrhea
One of the most important things you can do is keep feeding your baby. Breastfed babies should continue nursing, and in fact should nurse more often than usual. Breast milk contains protective antibodies that help fight infection, and the fluid and electrolyte balance is naturally suited for rehydration. Formula-fed babies should continue their regular formula unless a doctor advises switching.
For babies who have started solids, stick to bland, easy-to-digest foods like bananas, rice cereal, and applesauce. Avoid fruit juices, which contain sugars that can worsen diarrhea by pulling more water into the intestines. The goal is to maintain nutrition and hydration while the gut heals, which for most viral illnesses takes 3 to 7 days.
The World Health Organization recommends zinc supplementation during diarrhea episodes in children, at a dose of 10 mg per day for infants under six months and 20 mg per day for older babies, continued for 10 to 14 days. This has been shown to reduce the duration and severity of diarrhea. Ask your baby’s pediatrician whether zinc is appropriate for your child’s situation.