A first stomach bug in a baby is genuinely alarming — projectile vomit, watery green stools, a tiny person who suddenly seems floppy and miserable. Most of it is a self-limiting viral illness that clears in 3–7 days. The job at home is keeping fluids in, watching for dehydration, and knowing the handful of warning signs that mean this isn't a manage-at-home situation any more.
Logging the timeline as it happens — first vomit, number of loose stools, last wet nappy, any temperature — gives a healthcare professional something concrete to work with if you do call. Healthbooq has a simple log for exactly this.
What a Typical Bug Looks Like
Most viral gastroenteritis in babies and toddlers follows a recognisable shape. Sudden vomiting comes first, often forceful and frequent in the first 12–24 hours. Then it eases, while diarrhoea takes over and persists for several days. Total acute illness is usually 3–7 days. Loose stools sometimes hang around for up to two weeks afterwards as the gut recovers — this "post-gastroenteritis" pattern is normal as long as the child is otherwise fine and feeding well.
A low-grade fever (under 38.5°C) is common. Appetite drops. Mood is poor. None of that needs separate treatment.
Rotavirus typically produces watery green stools and significant vomiting; since the rotavirus vaccine entered the UK schedule in 2013 (oral drops at 8 and 12 weeks), severe rotavirus admissions have fallen sharply. Norovirus is the more common culprit now and famously sweeps through nurseries and households — it is exceptionally infectious. Hand washing with soap and water (alcohol gel doesn't reliably kill norovirus) is the main defence.
Fluids: The Whole Game
Hydration is the single most important thing you do at home.
Breastfed babies. Keep breastfeeding. Don't withhold the breast even briefly. Offer more often than usual — every hour or two if your baby will take it. Breast milk is the right fluid for a breastfed baby with gastroenteritis: hydrating, calorie-dense, and full of antibodies. Smaller, more frequent feeds tend to stay down better than long ones.
Formula-fed babies. Continue formula at normal strength. Do not dilute it — diluting reduces the sodium your baby needs and can cause low blood sodium, which is dangerous. Smaller, more frequent bottles work better than full feeds when there's vomiting.
Babies and toddlers with significant vomiting. Use oral rehydration solution (ORS) — Dioralyte, Electrolade, or supermarket equivalents. Mix according to packet instructions; don't dilute or concentrate. ORS works because of the gut's glucose-sodium co-transporter, which keeps absorbing fluid even in a battered intestine — water alone doesn't carry the sodium needed to actually rehydrate.
The technique that works for a vomiting toddler: 5–10 mL every 5 minutes with a syringe or teaspoon. Big drinks rebound. NICE specifically recommends this small-and-frequent approach for mild-to-moderate dehydration. Once they hold it down for 30 minutes, slowly build the volume.
What not to use as the main rehydration fluid: plain water (no sodium), fruit juice (too sugary, can worsen diarrhoea), squash, fizzy drinks, sports drinks (wrong electrolyte balance). For older toddlers, very dilute apple juice can be acceptable for mild dehydration if ORS is refused, but ORS is the standard.
Frozen ORS in ice-lolly moulds is a tactic worth knowing for a toddler who refuses to drink — many children will accept ice that they'll spit out as liquid.
Spotting Dehydration
This is what you are watching for. The signs run on a spectrum.
Mild dehydration: wet nappies a bit less often, mouth slightly dry, fewer tears, child still alert. Manage at home with more fluids.
Moderate dehydration: noticeably fewer wet nappies (under 4 in 24 hours in a baby; less than every 6–8 hours in a toddler), dry mouth and tongue, no tears when crying, eyes look slightly sunken, lethargic, irritable, skin pinched on the abdomen springs back slowly. Same-day GP or 111.
Severe dehydration (emergency, 999 or A&E): no wet nappy in 8+ hours, sunken fontanelle in a baby under 12 months, very dry mouth, deeply sunken eyes, mottled or cold hands and feet, hard to rouse or unusually drowsy, fast breathing, weak cry. This needs IV fluids.
The fontanelle (soft spot) is the simplest dehydration check in a baby. Sunken means trouble. Bulging is a different problem and also serious.
Food
Once the child is willing to eat, feed normally. The old BRAT diet (banana, rice, apple, toast) is out of date — both NICE and WHO recommend continuing usual food. Plainer carbs (toast, rice, pasta, potato, plain crackers) are easy to manage, but you don't need a special diet. Re-introducing milk and solids causes a brief loose-stool flare in some children but doesn't prolong the illness.
When to Seek Help
Same-day GP, 111 call, or A&E for any of:
- Any baby under 3 months with vomiting and/or diarrhoea
- A baby under 6 months who is losing fluid faster than they can take it in
- Bile-coloured (green or yellow) vomiting in a young infant — this can mean intestinal obstruction (intussusception, malrotation) and needs urgent assessment
- Blood or significant mucus in the stool
- Signs of moderate dehydration (above)
- Persistent vomiting where the child can't keep down any fluid
- Fever above 38°C in a baby under 3 months; above 39°C in an older child
- Symptoms worsening rather than improving after 48 hours
- Recent foreign travel with diarrhoea
- A baby or child who is unusually floppy, drowsy, or "just not right"
999 / A&E if:
- Severe dehydration (sunken fontanelle, no wet nappy 8+ hours, hard to rouse, mottled skin, unusually fast breathing)
- Sudden severe abdominal pain, drawing knees up, screaming spells with pallor (possible intussusception, especially in babies 3–24 months)
- Seizure
- Persistent green bile vomiting
A useful rule: if your gut tells you something more serious is going on, ring 111 or take them in. The cost of a false alarm is low; the cost of waiting on a serious bug is high.
What Not to Give
- Loperamide (Imodium): not for children under 12.
- Antibiotics: not for typical viral gastroenteritis. Used selectively for proven bacterial causes by clinicians.
- Anti-sickness medicines: not routinely given at home.
Going Back to Nursery
UKHSA guidance is 48 hours from the last episode of diarrhoea or vomiting before returning to nursery, school, or organised settings. Norovirus particularly continues to shed in stool for some time afterwards; the 48-hour rule is what stops it sweeping the room.
Key Takeaways
Gastroenteritis (diarrhoea and vomiting) is very common in babies and young children and is usually caused by a virus, most often rotavirus or norovirus. Most cases resolve within three to seven days without specific treatment. The primary management is preventing dehydration — maintaining adequate fluid intake, using oral rehydration solution (ORS) when vomiting is significant, and continuing breastfeeding. Babies under three months with gastroenteritis should be seen promptly by a healthcare professional. Signs of dehydration — reduced wet nappies, dry mouth, sunken fontanelle, lethargy — warrant medical assessment at any age.
