Symptoms guide

Fever

A fever is a body temperature that is higher than normal. It is usually a sign that the body is fighting an infection, and in most cases it is not harmful on its own.

Last updated 3 sources

At a Glance

  • In adults, a temperature of 100.4°F (38°C) or higher is generally considered a fever.
  • Fever is most often caused by viral or bacterial infections.
  • Fever is part of the immune response and is a symptom rather than a disease.
  • In infants younger than 3 months, any fever is generally treated as a reason for prompt medical evaluation.
  • Fever with a stiff neck, confusion, trouble breathing, or a seizure is generally treated as a medical emergency; in the US, people are advised to call 911.

What It Is

Normal body temperature varies from person to person and through the day, but it is often around 98.6°F (37°C). A fever happens when the brain raises the body's temperature set point, usually in response to substances released during an infection or inflammation.

Fever can make people feel cold and shivery as their temperature rises, then hot and sweaty as it falls. It often comes with tiredness, aches, and reduced appetite.

Most fevers go away within a few days as the underlying illness improves. However, fever can be more concerning in very young infants, older adults, people with weakened immune systems, and when it comes with certain other symptoms.

Common Terminology

Pyrexia
The medical term for fever.
Hyperthermia
A rise in body temperature caused by outside heat or the body failing to cool itself, such as heat stroke, rather than by a change in the brain's set point.
Febrile seizure
A seizure in a young child triggered by fever, usually between about 6 months and 5 years of age.
Antipyretic
A medicine that lowers fever.
Chills (rigors)
Shivering and feeling cold, often as body temperature rises.
Sepsis
A life-threatening reaction to infection that can damage organs and requires emergency treatment.

Common Causes and Associations

Fever is commonly associated with the following. This is general education, not a way to identify the cause of a particular fever.

  • Viral infections, such as colds, influenza, and COVID-19.
  • Bacterial infections, such as strep throat, ear infections, urinary tract infections, and pneumonia.
  • Inflammatory and autoimmune conditions, such as rheumatoid arthritis or inflammatory bowel disease.
  • Reactions to some vaccines, which can cause a short-lived, mild fever.
  • Certain medicines, which in some people can cause a drug-related fever.
  • Less commonly, some cancers or blood clots.

Symptoms and Features

This is a general description of how this symptom is often described, not a diagnostic checklist.

  • Feeling warm or hot to the touch, flushed skin, or sweating.
  • Chills and shivering, especially as the temperature rises.
  • Headache, muscle aches, tiredness, and weakness.
  • Loss of appetite and mild dehydration.
  • In young children, fussiness, poor feeding, or unusual sleepiness.

Tests and Evaluation

Clinicians generally look for the source of a fever through history, exam, and targeted tests.

  • Measuring temperature, commonly by mouth, ear, forehead, armpit, or rectum; rectal readings are generally considered most accurate in infants.
  • A physical exam looking at the throat, ears, lungs, skin, and abdomen.
  • Blood tests, such as a complete blood count, and blood cultures when a serious infection is suspected.
  • Urine tests to look for urinary tract infection.
  • Swab tests for infections such as flu, COVID-19, or strep throat, and imaging such as a chest X-ray in some cases.

Treatment Overview

Treatment usually focuses on comfort and on the underlying cause.

  • Rest and drinking enough fluids are commonly recommended to prevent dehydration.
  • Over-the-counter fever reducers such as acetaminophen or ibuprofen are commonly used for comfort, following label or clinician directions.
  • Aspirin is generally not given to children and teenagers because of the risk of Reye's syndrome.
  • Bacterial infections may be treated with antibiotics prescribed by a clinician; viral infections often get better on their own.
  • Light clothing and a comfortable room temperature can help with comfort.

When Professional Care May Be Discussed

Health organizations generally describe the following situations as reasons for urgent or prompt evaluation.

  • Fever with stiff neck, severe headache, confusion, a seizure, trouble breathing, or a rash that does not fade under pressure is generally treated as a medical emergency; in the US, people are advised to call 911.
  • Any fever in an infant younger than 3 months is generally treated as a reason for prompt medical evaluation.
  • Fever in people with weakened immune systems, such as during chemotherapy, is commonly evaluated promptly.
  • A fever that lasts more than a few days or keeps returning is generally discussed with a healthcare professional.
  • Signs of dehydration, such as very little urine or extreme drowsiness, are commonly treated as a reason for prompt care.

Questions to Ask a Healthcare Professional

General questions people often find useful to bring to an appointment:

  • What temperature is generally considered a fever in adults, children, and infants?
  • Which method of measuring temperature is most accurate at different ages?
  • How do clinicians tell whether a fever is from a viral or bacterial infection?
  • Why is aspirin generally avoided in children and teenagers?
  • What are febrile seizures and how are they generally managed?
  • What fever warning signs are considered emergencies?

Sources

This guide was written in plain language from the following authoritative references.

  1. Fever — MedlinePlus (U.S. National Library of Medicine)
  2. High temperature (fever) in adults — NHS (UK National Health Service)
  3. High temperature (fever) in children — NHS (UK National Health Service)

Last updated October 8, 2026. FreeDoc guides are general educational information and are not a substitute for care from a qualified professional.