General Health

Fever in Adults and Children — When It Becomes Dangerous

When a fever signals urgent care is needed in adults and children, and red flags to watch for.

Updated: 7 min read

Intro

A fever is a temporary rise in body temperature above the normal range — typically above 38°C (100.4°F). It is one of the body’s most effective defences: the higher temperature slows bacterial and viral replication while signalling the immune system to respond.

Most fevers in previously healthy people are caused by common infections and resolve within a few days with rest and fluids. However, some fevers — particularly in very young infants, older adults, people with weakened immune systems, or those with specific accompanying symptoms — can become life-threatening quickly. This guide helps you distinguish between a fever that can be managed at home and one that needs emergency attention.

Key Points

  • Fever is not a disease — it is the body’s response to infection or inflammation.
  • In infants under 3 months, any fever above 38°C (100.4°F) is a medical emergency until proven otherwise.
  • A non-blanching rash alongside fever is a red flag for meningococcal disease — call emergency services immediately.
  • High fever in adults above 40°C (104°F) with confusion or a severe headache requires urgent evaluation.
  • Do not bundle up a feverish person — keep them lightly dressed, cool, and hydrated.
  • Paracetamol or ibuprofen can reduce discomfort; they do not need to be alternated unless a clinician advises it.

When to Get Emergency Help

Adults — call emergency services or go to emergency care immediately if:

  • Temperature is above 40°C (104°F) and not responding to medication
  • Confusion, disorientation, or unusual behaviour
  • Severe headache and neck stiffness — possible meningitis
  • Difficulty breathing or rapid breathing
  • A rash that does not fade when pressed firmly with a clear glass — non-blanching rash
  • Persistent vomiting preventing any hydration
  • Fever that is worsening beyond 3 days despite treatment
  • Any fever in someone who is immunocompromised — on chemotherapy, long-term steroids, or with HIV

Children and infants — call emergency services or go to emergency care immediately if:

  • Any fever in a baby under 3 months old above 38°C (100.4°F) — regardless of how well they appear
  • Temperature above 39°C (102.2°F) in an infant aged 3–6 months
  • Seizure — any convulsion associated with fever
  • A rash that does not fade when pressed with a glass — non-blanching rash
  • Child is unresponsive, floppy, very difficult to wake, or not making eye contact
  • Breathing fast, appearing to work hard to breathe, or making a grunting sound
  • Persistent, inconsolable crying, a high-pitched unusual cry, or a bulging fontanelle
  • Pale, mottled, or blue-tinged skin, lips, or tongue

Common Causes

Fever is most often caused by:

Viral infections — the most common cause by far. Respiratory viruses (influenza, RSV, COVID-19, rhinovirus), gastroenteritis viruses, and many others cause fever as a normal immune response.

Bacterial infections — urinary tract infections, tonsillitis, pneumonia, ear infections, skin infections, and in serious cases, meningitis or bacteraemia. Bacterial infections often (but not always) cause higher, more persistent fevers.

Post-vaccination reaction — a mild fever within 24–48 hours of a vaccine is common, expected, and harmless. It indicates the immune system is responding. Manage with paracetamol if uncomfortable.

Heat exhaustion or heatstroke — prolonged exposure to hot environments can raise core body temperature. This requires cooling measures rather than antipyretic medication, and differs from infectious fever.

Inflammatory conditions — autoimmune disease flares, inflammatory bowel disease, and certain medications can cause fever without infection.

Teething — causes only a very slight temperature rise in infants, not a true fever. A high temperature in a teething baby should be evaluated like any other fever.

What to Do Right Now

For any fever — at home steps:

  1. Monitor temperature using an accurate thermometer. Axillary (armpit), oral, or ear thermometers are reliable. Forehead adhesive strips are less accurate.
  2. Keep the person hydrated — small, frequent sips of water, or oral rehydration solution if vomiting. Continue breastfeeding or formula feeding in infants as normal, with extra feeds if tolerated.
  3. Dress lightly — remove heavy layers. Keep the room at a comfortable temperature around 18–20°C. Do not wrap in blankets.
  4. Give fever-reducing medicine if the person is uncomfortable — use the correct dose for age and weight.
  5. Check for red-flag symptoms regularly — especially perform the glass test on any new rash.

The glass test — do this for any rash alongside fever:

Press a clear glass firmly against the rash. If the rash fades (blanches) under pressure, it is less likely to indicate meningococcal disease. If it does not fade, call emergency services immediately — this is a medical emergency that cannot wait.

Fever-reducing medicines:

Paracetamol (acetaminophen): Safe for adults and children from birth. Follow package dosing by weight in children, not by age alone. It can be given when the person is uncomfortable, even if the temperature is not extremely high.

Ibuprofen: Effective for reducing fever and easing pain and inflammation. Suitable for adults and children over 6 months of age. Avoid in dehydration, suspected chickenpox, kidney problems, stomach ulcers, and most heart conditions. Do not give to children with suspected dehydration without medical guidance.

Do not give aspirin to children or teenagers — it carries a risk of Reye’s syndrome, a rare but serious condition.

On alternating medicines: Using paracetamol and ibuprofen at staggered intervals can be helpful in some situations, but is not routinely needed. Never give both at the same time. Always follow the recommended dose and timing.

FAQ

What temperature is a fever in adults? Above 38°C (100.4°F) is generally considered a fever. High fever is above 39.4°C (103°F). A temperature above 40°C (104°F) that is not responding to medication, or one accompanied by confusion, severe headache, or a non-blanching rash, requires urgent emergency evaluation.

When should I take a baby to the emergency department? Any fever in a baby under 3 months old (above 38°C / 100.4°F) should be assessed urgently — even if the baby appears well. At this age, serious infections can progress rapidly and may show very few outward signs.

Should I try to sweat out a fever? No. Heavy blankets trap heat and can cause dangerous overheating. Keep the person lightly dressed in a cool room and ensure they take regular fluids.

What is the glass test and when should I use it? Press a clear glass firmly against any rash. If the rash does not fade with sustained firm pressure, it may indicate meningococcal septicaemia — a life-threatening emergency. Call emergency services immediately if the rash does not blanch.

What is a febrile seizure? A convulsion triggered by a rapidly rising fever in children aged 6 months to 5 years. Most last under 5 minutes and do not cause lasting harm — but call emergency services for any first seizure or one lasting more than 5 minutes. Do not restrain the child; protect them from injury and note how long it lasts.

Can I alternate paracetamol and ibuprofen? You can use one or the other as needed. Alternating is sometimes recommended by clinicians but is not routine standard practice. Never give both at the same time, and always follow dosing instructions for the child’s weight.

How long should a fever last? Most viral fevers resolve within 3–5 days. A fever that lasts more than 3 days, keeps rising despite medication, or is associated with worsening symptoms should be evaluated by a clinician. Bacterial infections often require antibiotic treatment.

Is fever dangerous in immunocompromised people? Yes. People on chemotherapy, immune-suppressing medications, or with HIV should treat any fever — even a low-grade one — as potentially serious and seek prompt medical assessment.

Further Reading