The first time you see your toddler's mouth covered in painful ulcers, with fever and dribble running down their chin because swallowing hurts, it looks much worse than the small lip blister that "cold sore" usually brings to mind. This is primary herpes simplex infection — most children's first encounter with a virus that two-thirds of adults already carry, often without realising.
In healthy children beyond the newborn period, it's miserable but rarely dangerous. The two situations that change the rules are very young babies and any involvement near the eye.
Healthbooq covers common childhood infections and the practical questions that come up at home.
What's Behind a Cold Sore
Cold sores are caused by herpes simplex virus type 1 (HSV-1). The WHO estimates that around 2 in 3 adults under 50 worldwide carry it. Most acquire it in childhood, usually from a kiss from a parent, grandparent, or other relative — often someone who didn't have a visible cold sore at the time, because the virus can shed even when no sore is present.
Once you have it, you have it for life. The virus retreats into nerve cells (the trigeminal ganglion, near the side of the face) and stays there indefinitely, reactivating from time to time to produce the familiar lip blister. It cannot be eliminated.
This is uncomfortable knowledge when it's just hit your toddler's mouth, but it isn't anyone's fault. The virus is everywhere, and most adults shedding it don't know they're doing so.
The First Infection: Gingivostomatitis
Most children's first encounter with HSV-1 is between roughly 6 months and 5 years. About a third have a noticeably symptomatic primary infection; the rest pass through it with no recognisable illness or with a few unremarkable mouth ulcers.
When it is symptomatic, it goes by the name primary herpetic gingivostomatitis. The picture:
- Painful mouth ulcers on the gums, inner cheeks, tongue, lips, and around the mouth. Often clusters of blisters that quickly break to leave shallow ulcers.
- Bleeding gums that look raw and inflamed.
- Drooling because swallowing hurts and the child is producing extra saliva.
- Refusal to eat or drink — often the most concerning thing for parents.
- Fever, often 38.5–40°C, for several days.
- Swollen, tender lymph nodes in the neck.
- Irritability and sleep disturbance because the mouth genuinely hurts.
It looks much worse than the typical adult cold sore. Many parents take their child to A&E thinking it must be hand-foot-and-mouth disease or something more sinister. The illness usually lasts 7 to 10 days, with the worst at days 3–5.
What Helps
The illness is self-limiting in healthy children. Your job is to keep them comfortable and hydrated.
For pain and fever:
- Paracetamol and ibuprofen on rotation at age-appropriate doses, regularly rather than only when the child complains. Pain relief makes the difference between a child who refuses fluids and one who will drink.
- Topical gels (children's teething gels with a numbing component, like Calgel-style products, or anaesthetic mouth gels) can help if the child will tolerate them being applied. Avoid in babies under specific age cutoffs as recommended on the product label.
For hydration:
- Cold things are better than warm. Ice lollies, frozen yoghurt, ice chips. The cold numbs the mouth and the calories help.
- Soft, cold, bland foods — yoghurt, custard, ice cream, smooth mashed potato, cold mashed banana.
- Avoid acidic foods and drinks — orange juice, tomato, pineapple. They sting badly on broken mucosa.
- Frequent small sips rather than waiting for a thirst that may not come. Use a small cup or syringe if needed.
Antiviral treatment:
- Oral aciclovir, prescribed by a GP, can shorten the illness and reduce viral load if started within 72 hours of symptom onset. Whether to use it for routine cases is a clinical judgement — many GPs reserve it for more severe infections, very young children, or where the illness has interfered significantly with intake. If your child seems particularly unwell, ask.
When to See a Doctor During First Infection
Most cases can be managed at home. See a GP or out-of-hours service if:
- Your child cannot maintain enough fluid to keep producing wet nappies (under 4 hours between wet nappies in a young child is concerning).
- Signs of dehydration: very dry mouth, sunken eyes, lethargy, no tears when crying.
- Fever above 39°C that's not coming down with paracetamol.
- The illness extends beyond 14 days.
- The child seems significantly unwell beyond the mouth — drowsy, floppy, hard to rouse.
- Any spread of the rash or blisters far beyond the mouth.
Recurrent Cold Sores
After the first infection, the virus is dormant. Reactivations look different — much smaller, much less unpleasant.
The classic sequence:
- Tingling or burning on or near the lip, often hours before anything visible. Older children can describe this as it happens.
- Cluster of small blisters appears, usually on the lip border or just outside it.
- Blisters break, weep, and crust over within 2–4 days.
- Crust falls off and skin heals by 7–10 days.
Common triggers:
- A cold or other illness with fever (the original name "cold sore" comes from this).
- Sunburn or strong UV exposure.
- Fatigue, stress, exam pressure in older children.
- Hormonal changes around menstruation in adolescents.
- Local trauma to the lip (sport, dental work).
Treatment of Recurrences
Over-the-counter aciclovir cream (Zovirax and others) is most useful when applied at the first tingle, before the blister forms. It can shorten the outbreak by a day or so. Once the blister has crusted, it's largely cosmetic. Apply five times a day for five days.
Cold compresses, lip balm to prevent splitting, and avoiding picking the crust all speed healing.
For recurrent severe outbreaks — six or more episodes a year, or outbreaks affecting school or quality of life — a GP can prescribe suppressive oral aciclovir taken daily for several months.
Contagious Period and Hygiene
Cold sores are contagious from the first tingling until the scab has fully fallen off and skin is healed. Practical implications:
- No kissing on the lips or face. Including parents kissing children with active sores, and especially kissing newborns.
- No sharing cups, water bottles, cutlery, lip balms, or towels.
- Wash hands after touching the sore (and ideally avoid touching it at all).
- Don't pop blisters — increases viral spread to other parts of the body.
A child with a small recurrent cold sore can usually go to nursery or school. Active gingivostomatitis with fever and severe symptoms generally means staying home until they feel better and can manage food and drink.
Newborns: Different Rules
Neonatal herpes — HSV infection in a baby under 4 weeks — is rare (around 1 in 60,000 UK live births) but can be devastating. Without treatment, it can disseminate to the liver, lungs, and brain, with high mortality. Even with treatment, neurological damage is common in disseminated cases.
A newborn's immune system cannot contain HSV the way an older child's can.
Urgent action for any baby under 4 weeks with:
- Blisters or sores anywhere on the skin or mouth.
- Fever (≥38°C in a baby under 3 months is always a paediatric emergency, regardless of cause).
- Lethargy, poor feeding, irritability, or breathing change.
- Eye redness or discharge.
→ Take the baby to A&E or call 999 if very unwell. Do not wait for a GP appointment.
If a newborn has been kissed by, or had close contact with, someone who had an active cold sore, mention it specifically to the midwife or GP — even if the baby seems well. Antiviral treatment may be considered.
The biggest preventive measure is the simplest: don't kiss a newborn if you have a cold sore, and don't let visitors with active cold sores hold or kiss the baby. Most cases of neonatal herpes come from contact with an adult who didn't realise they were infectious.
Eye Involvement: An Emergency Outside the Newborn Period Too
HSV-1 can infect the cornea of the eye, causing HSV keratitis. If untreated, it can scar the cornea and impair vision permanently.
Signs:
- Red, painful, watery eye, sometimes with a cold sore nearby on the face.
- Light sensitivity.
- Blurred vision.
- A small "branch-shaped" ulcer visible on the cornea (the GP or A&E doctor may see this with a fluorescein stain).
If a child has a red painful eye and there is a cold sore on their face, or has had one recently, this needs same-day assessment — through urgent care, A&E, or an eye casualty service. Treatment is with topical aciclovir eye ointment and sometimes oral aciclovir.
Children should also be discouraged from rubbing their eye with hands that have touched a cold sore, especially during contact lens wear in older children.
Children with Weakened Immune Systems
Cold sores in children on chemotherapy, on long-term immune-suppressing medication, with HIV, or with conditions like atopic eczema (where extensive skin involvement — eczema herpeticum — can occur) need a lower threshold for medical attention. Eczema herpeticum is a particularly important one to recognise: a child with eczema who develops crops of small punched-out lesions, fever, and feels unwell needs same-day medical assessment and usually oral or IV aciclovir.
Key Takeaways
Cold sores are caused by herpes simplex type 1, a virus most adults carry. The first infection in a child — gingivostomatitis — is often dramatic, with painful mouth ulcers, fever, and refusal to eat or drink for a week or more. The main risk is dehydration, not the virus itself. After the first infection the virus stays in the body for life and reappears occasionally as the familiar lip blister. Two situations need urgent medical attention: any cold-sore-like blisters in a baby under 4 weeks (neonatal herpes is a medical emergency) and a red painful eye anywhere near a cold sore (risk of corneal damage).
