Fever

Key takeaways

• Commonly defined as: 100.4°F (38°C) or higher
• In babies under 3 months: 100.4°F (38°C) or higher needs a call to the doctor right away
• Most useful single change: Keep fluids up; this matters more than lowering the temperature.
• Commonest mistake: Alternating acetaminophen and ibuprofen routinely without advice.
• See a doctor if any of the signs under "When to get help" below apply to you.

Why it matters

It is a sign rather than a disease, and the number matters far less than how the person looks and behaves.

The numbers, in one place

Published figures for Fever

Measure	Figure
Commonly defined as	100.4°F (38°C) or higher
In babies under 3 months	100.4°F (38°C) or higher needs a call to the doctor right away
Acetaminophen (Tylenol) and ibuprofen	for comfort, not to bring the number back to normal
Ibuprofen	not for babies under 6 months unless a doctor advises it
Do not give aspirin	to children or teenagers
Dehydration	the main practical risk in most fevers

What helps

• Keep fluids up; this matters more than lowering the temperature
• Treat for comfort rather than to reach a target reading
• Dress lightly rather than bundling up or sponging with cold water
• Check on the person regularly, including overnight

What to avoid

• Alternating acetaminophen and ibuprofen routinely without advice
• Aspirin in children and teenagers
• Judging severity by the number alone

In depth

Body temperature is controlled by the hypothalamus, a region of the brain that acts like a thermostat. During an infection, immune cells release signaling molecules that reset that thermostat to a higher point. Your body then works to reach it, which is why you feel cold and shiver as a fever rises, even though your temperature is going up. When the infection eases or a fever reducer takes effect, the set point drops and you sweat and feel hot as your body cools.

A raised temperature helps the immune response in several ways. Some germs multiply more slowly at higher temperatures, and certain immune cells work more efficiently. This is why fever is generally thought of as part of the defense, not the enemy. Most fevers from common infections are not harmful in themselves. Very high body temperatures from heatstroke are a different situation, because the body's cooling system has failed rather than been deliberately reset.

What matters most is the cause of the fever and how the person is doing. A child with a high fever who is playing and drinking between doses of medicine is often less concerning than one with a lower reading who is limp, unusually drowsy or struggling to breathe. Age changes the picture: in babies under 3 months, any fever of 100.4°F (38°C) or higher needs prompt medical assessment because serious infections can look mild at first.

Signs and symptoms

• Feeling hot or flushed, or cold and shivery as the temperature rises
• Sweating as the fever breaks
• Headache, muscle aches and tiredness
• Loss of appetite and reduced thirst, which increases the risk of dehydration
• In children, irritability, crying more than usual or being sleepy
• Faster heart rate and breathing while the temperature is high

Causes and risk factors

• Viral infections such as colds, flu and COVID-19, the most common cause
• Bacterial infections such as urinary tract infections, pneumonia and ear infections
• Inflammatory and autoimmune conditions
• Reactions to some medicines and, briefly, after some vaccines
• Less commonly, certain cancers and blood disorders, especially with prolonged unexplained fever
• Heat-related illness, which raises temperature by a different mechanism

How it is checked

An accurate reading starts with the right thermometer and method. For babies and young children, a rectal digital thermometer gives the most reliable reading; for older children and adults, oral readings work well. Forehead and ear thermometers are convenient, but their accuracy depends on correct technique, so follow the instructions. Avoid mercury thermometers, and note that touching the forehead is not reliable.

When you see your doctor, they will look for the source of the fever by asking about other symptoms and examining the ears, throat, chest, skin and abdomen. Depending on age and findings, tests may include a urine sample, blood tests, a swab for flu or COVID-19, or a chest X-ray. Babies under 3 months with fever often need blood and urine tests and sometimes more, because they can have serious infections without obvious signs.

Related topics

• Fever in Children: What Published Guidance Supports (Children's Health)
• Safe Internal Cooking Temperatures: What Published Guidance Supports (Food Safety)
• Heat Exhaustion and Heat Stroke: What the Evidence Actually Says (Heat, Cold and Weather Safety)
• Acetaminophen Safety: What the Evidence Actually Says (Liver Health)

Where to go next

• What People Get Wrong About Fever
• Starting Out With Fever
• Immunity: The Complete Guide
• Vitamin C: What the Evidence Actually Says
• Zinc: What the Evidence Actually Says

When to get help

Call 911 for a rash that does not fade under pressure, a stiff neck with sensitivity to light, a seizure, difficulty breathing, or a child who is limp, will not wake or is unusually drowsy. Any fever of 100.4°F (38°C) or higher in a baby under 3 months needs a call to the doctor right away or a trip to the emergency room. Get prompt advice for fever in anyone with a weakened immune system or having chemotherapy.

Where this comes from

• American Academy of Pediatrics — Fever and Your Baby
• MedlinePlus — Fever

How this page was put together

Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarized or generalized beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.

Medical disclaimer

This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to your doctor, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 911.