When to get help

Call an urgent advice line today — NHS 111 in the UK, the GP out-of-hours service in Ireland, your paediatrician in the US or Canada, healthdirect on 1800 022 222 in Australia, Healthline on 0800 611 116 in New Zealand — if:

you are worried about a baby under 12 months. The NHS lists this on its own, with no other qualifier;
your baby stops breast or bottle feeding while they are ill;
there are signs of dehydration, such as fewer wet nappies;
your baby keeps being sick and cannot keep fluid down;
there is bloody diarrhoea or bleeding from the bottom;
diarrhoea lasts more than 7 days, or vomiting more than 2 days.

Call emergency services — 999 in the UK, 112 or 999 in Ireland and the EU, 911 in the US and Canada, 000 in Australia, 111 in New Zealand — if your baby vomits blood or vomit that looks like ground coffee, has yellow-green or green vomit, may have swallowed something poisonous, has a stiff neck and pain looking at bright lights, has a sudden severe tummy ache, has blue, grey, pale or blotchy skin, lips or tongue, is having severe difficulty breathing or taking quick short breaths, or is confused or not responding as usual.

Green vomit in a child is on the emergency list for a reason and is worth knowing before you see it.

Checking for dehydration in a baby

This is the whole job. The NHS lists the signs in a baby as:

a sunken soft spot (fontanelle) on top of the head;
sunken eyes;
few or no tears when they cry;
fewer wet nappies than usual;
being drowsy or irritable.

NICE guideline CG84 covers gastroenteritis in the under-5s for clinicians and treats dehydration assessment as the central task. Raising Children Network in Australia frames it the same way for parents. Wet nappies are the most usable measure at home because you can count them: note how many your baby normally has in 24 hours, and compare.

How long it should last

The NHS gives the timeline: diarrhoea usually stops within 5 to 7 days, and vomiting usually stops in 1 or 2 days. Anything beyond that is a reason to call. The WHO defines diarrhoea as three or more loose or liquid stools a day, or more frequent passage than is normal for that person — and adds a caveat that matters enormously for this site: the passing of loose, pasty stools by breastfed babies is not diarrhoea. Neither is frequent passing of formed stools.

If you are used to a breastfed baby's normal output, you already know this. If you are not, it is a common source of unnecessary alarm.

What to give

Keep feeding. Carry on breast or bottle feeding. If your baby is being sick, give smaller feeds more often than usual.
Small sips of water between feeds for babies on formula or solid foods. For a baby under six months, boil the water first and let it cool.
Oral rehydration solution if advised. A pharmacist can recommend one suitable for your baby's age. The WHO describes ORS as the standard treatment for diarrhoea — a solution of clean water, sugar and salt that replaces what is lost.
Keep giving food to babies who are eating solids, once they want it. The American Academy of Pediatrics advises against withholding food for longer than 24 hours, because a child needs nutrition to regain strength.

The AAP also notes that exclusively breastfed babies are less likely to develop severe diarrhoea, and that many breastfed babies stay hydrated on frequent breastfeeding alone.

What not to give

Not fruit juice or fizzy drinks. Both the NHS and AAP say these can make diarrhoea worse.
Do not make formula weaker. Use it at its usual strength. This is a surprisingly common mistake and it makes things worse, not gentler.
No anti-diarrhoea medicine. The NHS says not to give children under 12 medicine to stop diarrhoea; loperamide is specifically not suitable under 12.
No aspirin for children under 16.

Recovery, and what looks like a relapse but is not

The AAP describes the pattern well: after you restart food, stools may stay loose for a while, and that does not mean things are going badly. Look for increased activity, better appetite, more frequent weeing and the disappearance of dehydration signs. Those are the markers of recovery, not stool consistency.

Stopping it spreading

Stomach bugs spread easily. Wash hands with soap and water frequently. Wash any clothing or bedding with poo or vomit on it separately on a hot wash. Clean toilet seats, flush handles, taps, surfaces and door handles every day. Avoid preparing food for other people. Do not share towels, flannels or cutlery.

The exclusion rule the NHS gives is to stay off school, nursery or work until you have not been sick or had diarrhoea for at least 48 hours, and not to use a swimming pool until at least 48 hours after symptoms stop.

Why this matters more than it feels like it should

The WHO describes diarrhoeal disease as the third leading cause of death in children aged 1 to 59 months, and notes that it is both preventable and treatable. That statistic is driven by settings without safe water or rapid access to care, and it is not a prediction about a baby with a winter tummy bug in a country with a health service. It is context for why the fluid advice is so insistent.

Gastroenteritis and Dehydration