Hand, foot and mouth, RSV, and other common baby viruses

Babies get a lot of viruses in the first two years of life — up to 8–12 colds per year alone. Most are self-limiting and resolve with supportive care. A handful can escalate quickly, especially in young infants. This guide covers the most common ones, what to watch for, and when to stop waiting and call for help.

Hand, foot and mouth disease (HFMD)

Cause: Usually coxsackievirus A16, sometimes enterovirus 71 or other enteroviruses.

Who gets it: Most commonly children under 5. Very common in nurseries and groups.

What it looks like:

1. Fever (often the first sign) and feeling generally unwell for 1–2 days
2. Mouth ulcers — small, painful sores on the tongue, gums, and inside the cheeks; these make eating and drinking uncomfortable
3. Rash on hands and feet — small flat red spots or tiny blisters on the palms, soles, and sometimes the knees, elbows, or buttocks

Duration: Typically 7–10 days. Highly contagious through saliva, blister fluid, and stool for up to several weeks.¹

What to do at home:

• Offer cool, soft foods — yoghurt, ice lollies, cold milk; avoid acidic or salty foods that sting the mouth ulcers
• Give adequate fluids; the mouth pain can lead to refusing drinks, which risks dehydration
• Infant paracetamol at the appropriate dose can help with fever and pain
• Keep your baby away from other children and pregnant women until blisters have crusted

When to go to the emergency department:

• Signs of dehydration — see Dehydration in babies for the severity signs
• Your baby is under 3 months with any HFMD symptoms
• Seizures (rare complication, more associated with enterovirus 71)
• Rapidly worsening unwell behaviour

RSV (respiratory syncytial virus) and bronchiolitis

RSV is the most common cause of bronchiolitis — an inflammation of the small airways in the lungs — and is the leading cause of hospital admission in infants under 1 year in the UK and US.²³

Season: Usually October to March in the northern hemisphere; peak December–February.

Who is most at risk: All infants can get RSV, but severe illness is most common in babies under 6 months, premature babies, and those with heart or lung conditions.

What it looks like:

• Starts like a cold — runny nose, mild cough, low fever — for 2–3 days
• Progresses to a wetter, persistent cough
• In some babies, this progresses further to bronchiolitis: fast breathing, chest pulling in (retractions), and difficulty feeding

Normal breathing rate for reference: Under 2 months, up to 60 breaths/minute is normal. If you're unsure, count breaths for a full minute while your baby is calm.

When to call 999 / go to A&E immediately:²³

• Breathing rate persistently above 70 breaths/minute in a baby under 1 year
• Retractions (skin pulling in between ribs) visible with breathing
• Grunting with each breath
• Nostrils flaring with each breath
• Blue or grey lips, tongue, or fingernails (cyanosis)
• Your baby is unusually limp or unresponsive
• Breathing pauses (apneas)

When to call your GP or 111 same day:

• Your baby is under 6 months with RSV symptoms worsening beyond a simple cold
• Your baby is refusing more than half of their usual feeds
• You're concerned about any change in breathing, even if not at the emergency threshold above

At home (mild RSV/cold symptoms): Keep nasal passages clear with saline drops and a bulb aspirator before feeds if congestion is making feeding difficult. Offer frequent shorter feeds. Keep your baby upright after feeds. Do not use decongestant medicines or cold remedies in babies under 2 years — these are not safe or effective.³

Croup

Cause: Usually parainfluenza virus (types 1 and 2).

What it sounds like: The classic "barking seal" cough — harsh, resonant, and often sudden in onset, frequently waking babies from sleep at night. It is caused by inflammation and swelling of the larynx (voice box) and trachea.⁴

Season: Most common autumn and winter.

Mild croup at home:

• Sit with your baby in an upright position — this eases the work of breathing
• Keep calm; anxiety worsens airway spasm
• Cool night air (opening a window, or briefly going outside on a cool night) can provide temporary relief

When to call 999 / go to A&E immediately:⁴

• Stridor at rest — a high-pitched, musical noise on breathing in while the baby is calm. Stridor only when upset or coughing is less alarming; stridor at rest indicates significant airway narrowing and needs emergency assessment
• Severe retractions (skin pulling in at the neck or chest)
• Your baby looks blue or very pale
• Drooling, difficulty swallowing, or abnormal posture (not wanting to lie down, leaning forward) — these suggest epiglottitis, a rarer but more serious condition
• Your baby is unusually agitated or exhausted

Croup can sometimes be treated with a single dose of oral steroid (dexamethasone or prednisolone) given by a doctor, which reduces airway swelling significantly within 30–60 minutes.⁴

The common cold

Babies average 6–12 colds in their first year of life as they encounter new viruses for the first time.

What to do: Saline drops and a bulb aspirator to clear congestion before feeds. Age-appropriate paracetamol for fever and discomfort. Offer feeds frequently. Expect it to last 7–10 days.

When to call your GP:

• Fever in a baby under 3 months (38°C/100.4°F or above — same-day assessment)
• Symptoms worsening significantly after day 5–7 (possible secondary bacterial infection — ear infection, chest infection)
• Breathing changes at any stage

Flu (influenza)

Flu is more severe than a cold: high fever (39°C/102.2°F+), muscle aches, extreme fatigue, cough, and sometimes vomiting. Babies under 6 months are at higher risk of complications including pneumonia.

Annual flu vaccination is recommended in the UK from age 2 and in the US from 6 months — see The baby vaccination schedule.

Call 999 / go to A&E if: Difficulty breathing, severe chest pain, confusion, inability to rouse, or symptoms rapidly deteriorating.

Vomiting and diarrhoea (gastroenteritis)

Common causes: Rotavirus (partially prevented by vaccination in UK and US), norovirus, adenovirus, and others.

What to do: The main risk is dehydration — see Dehydration in babies for the full severity guide. Continue breastfeeding. Offer ORS if your GP advises it. Do not give anti-diarrhoeal or anti-vomiting medications to babies without medical advice.

When to call 999 immediately: Signs of severe dehydration (no wet nappy in 8+ hours, limp or unresponsive, cold extremities), blood in stool or vomit, bile-green vomiting in a young infant (possible bowel obstruction).

Symptomatic care: the principles

Most baby viruses require the same supportive approach:

1. Maintain hydration — prioritise fluids over solids; breastfeed frequently
2. Manage fever — age-appropriate paracetamol at the correct dose; never aspirin
3. Keep nasal passages clear — saline drops before feeds
4. Rest — let your baby sleep as much as they need
5. Monitor — know the escalation signs for each illness and be ready to act

← Back to the complete guide: Baby health basics

Also in this cluster: Dehydration in babies · When to call the doctor · Baby fever: when to worry

Sources

1. NHS. "Hand, Foot and Mouth Disease." NHS, 2023. https://www.nhs.uk/conditions/hand-foot-mouth-disease/
2. NHS. "Bronchiolitis." NHS, 2023. https://www.nhs.uk/conditions/bronchiolitis/
3. CDC. "RSV (Respiratory Syncytial Virus)." Centers for Disease Control and Prevention, 2024. https://www.cdc.gov/rsv/
4. NHS. "Croup." NHS, 2023. https://www.nhs.uk/conditions/croup/