What Causes a High-Grade Fever and When Is It Too High?

If you are an adult with a fever over 103 F (39.4 C), you need to call your healthcare provider immediately. If it is over 105 F (40.5 C), you need to rush to your nearest emergency room. You may need to act even sooner when infants, children, older adults, and immunocompromised people have a fever.

It is important to know when a fever is too high because it can be dangerous to the central nervous system. Febrile seizures can occur in babies and young children with temperatures greater than 100.4 F. A sustained temperature greater than 107.6 F can even lead to brain damage.

This article will explain what fevers are and how to know when a fever is too high. It will also describe how fever is treated and when it's time to seek emergency medical care.

Why You Get Fever

A fever (also known as pyrexia) typically occurs when your body’s immune system is fighting an infection from a bacteria, virus, or other disease-causing pathogen. The fever helps neutralize the pathogen by raising the body temperature to where it cannot survive.

A high fever can also be caused by drug reactions, inflammatory diseases, autoimmune disorders, excess heat exposure, and other non-infectious conditions. These conditions cause fever by disrupting the part of the brain that functions as the body's thermostat, called the hypothalamus.

When using an oral thermometer, 98.6 degrees Fahrenheit (F) is considered a normal body temperature. Anything above 99.1 F is considered a fever in adults and children.

A fever is considered high when the temperature is above 103 F in adults. In young children and others, a high fever may be defined differently.

Causes of Fevers — Verywell / Jiaqi Zhou

Causes of High-Grade Fever

Almost any infection can cause a high-grade fever, particularly if the body has not been exposed to the pathogen before and has no immunity to it. A high fever can also occur if certain infections are left untreated and either get worse or start to affect other organs.

Infectious causes of high fever include:

Appendicitis (inflammation of the appendix)
Cellulitis (a skin infection in deeper skin layers)
Encephalitis (inflammation of the brain)
Meningitis (inflammation of the tissues surrounding the brain and spinal cord)
Mononucleosis (an infection mostly associated with the Epstein-Barr virus)
Osteomyelitis (a bone infection)
Respiratory infections like flu, COVID-19, pneumonia, and tuberculosis
Urinary tract infections

Some non-infectious conditions associated with high-grade fever include:

Intracranial hemorrhage (bleeding within the skull)
Thyroid storm (a condition where the thyroid gland makes too much thyroid hormones)
Serotonin syndrome (a type of serious drug reaction)
Sepsis (the body's overreaction to an infection that has spread to the bloodstream)
Kawasaki syndrome (a rare inflammatory disease in children)
Neuroleptic malignant syndrome (an adverse reaction to antipsychotic drugs)
Heatstroke (the dysregulation of body temperature to excess heat exposure)

What Are the Symptoms of High Fever?

The symptoms of fever get worse as the temperature rises. Fevers are broadly categorized as being low-grade, moderate-grade, high-grade, and hyperpyrexia.

Low-Grade Fever

A low-grade fever is defined as an oral temperature of 99.1 to 100.4 F (37.3 to 38 C) in adults.

Symptoms include:

Feeling hot
Headache
Achy eyes
Thirst
Loss of appetite

Moderate-Grade Fever

Moderate-grade fever is defined as an oral temperature of 100.6 to 102.2 F (38.1 to 39.0 C) in adults.

Symptoms include those listed above, plus:

Sweating
Dizziness or lightheadedness
Muscle and body aches
Fatigue
Nausea

High-Grade Fever

High-grade fever is defined as an oral temperature of 102.4 to 105.8 F (39.1 to 41 C) in adults.

It can cause all of the above, plus:

Chills and shivering
Extreme weakness
Increased heart rate
Pale skin
Stomach upset
Vomiting
Decreased urine

Hyperpyrexia

Hyperpyrexia is the term for an exceptionally high fever over 106.7 F (41.5 C). It can occur in people with a severe infection or a brain hemorrhage.

Symptoms include:

Extreme confusion
Hallucinations
Loss of consciousness
Shallow, rapid breathing
Hot, dry, red skin
Rapid but weak heartbeat
Dilated pupils
Inability to urinate
Seizures

Can a High Fever Kill You?

Although persistent high fever can lead to brain damage and death, this is rare and more often associated with the underlying cause of the fever, such as a malignancy (cancer).

Brain damage usually only occurs with a sustained fever over 107.6 F. The risk of death only becomes serious, hovering between 50% and 65%, when admission to an intensive care unit (ICU) is needed.

When to Act on a High Fever

Symptoms play as much of a role as a person's temperature in determining whether it is time to seek medical care for a fever.

For smaller children and babies, it's best to err on the side of caution and act when symptoms worry you, no matter what the thermometer says. The same applies to older adults and immunocompromised people.

With that said, you need to go straight to the emergency room when temperatures exceed the following in children and adults:

Children 3 months old or younger: A rectal temperature of 100.4 F or higher
Children 3 to 12 months: An oral temperature of 102.2 F
Children 2 years or younger: A fever that lasts longer than 24 to 48 hours
Older children, teens, and adults: A fever higher than 105 F, or a fever over 103 F that rises or lasts longer than 48 hours

Symptoms That Need Emergency Care

No matter how high a fever is, call 911 or rush to the nearest emergency room if a fever is accompanied by the following severe signs and symptoms:

Stiff neck with severe headache
Sudden, extreme sensitivity to light (photophobia)
Convulsions or seizures (febrile seizures)
Confusion and disorientation
Difficulty waking or staying awake
Difficulty breathing or shortness of breath
Severe persistent vomiting
A purplish or blood-colored rash (roseola rash)
Inability to urinate (urinary retention)

How to Reduce a Fever

If someone has a fever but feels fine for the most part, treatment isn't necessary. In fact, because the fever's job is to help neutralize an infection, treating a low-grade fever can interfere with the body's efforts.

When it comes to children who have a fever, a rule of thumb is that if they're playing and have energy, there is no immediate danger.

When fever-related symptoms are making you feel lousy or the temperature has crept up near the 103 F mark in adults and 102 F in children, an over-the-counter fever reducer like Tylenol (acetaminophen) or Advil (ibuprofen) can provide short-term relief.

Rest and plenty of fluids can help your body fight the underlying infection. A lukewarm bath can also help, but avoid ice baths which can cause shivering and raise body temperature even further.

Warning to Parents

Children and teenagers should never be given aspirin for a fever. Doing so increases the risk of Reye's syndrome, a rare and potentially life-threatening reaction that causes brain and liver damage.

Summary

A fever is one of the ways that the body fights infections. However, if a fever gets to 103 F or more in adults, you need to call a healthcare provider.

If a fever reaches 105 F or more, seek immediate emergency care. The same applies to babies 3 months or younger with 100 F, 3 to 12 months with 102 F, and 2 years or younger with fevers longer than 48 hours.

A Word From Verywell

Having a high fever can be a serious condition. Keep things like medication side effects, vaccinations, travel history, and environmental factors in mind so you are well-equipped to deal with one. Remember to always act promptly when dealing with a high fever to prevent complications.

— KASHIF J. PIRACHA, MD, MEDICAL EXPERT BOARD

13 Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

Leung AKC, Leung TNH. Febrile seizures: an overview. Drugs Context. 2018;7:212536. doi:10.7573/dic.212536
MedlinePlus. Fever.
Walter EJ, Hanna-Jumma S, Carraretto M, Forni L. The pathophysiological basis and consequences of fever. Crit Care. 2016;20(1):200. doi:10.1186/s13054-016-1375-5
National Health Service (UK). High temperature (fever) In adults.
University of Pittsburgh Medical Center. The dangers of high fever.
Harvard Medical School. Fever in adults: when to worry.
Schmutzhard E, Pfausle B. Hyperpyrexia in life-threatening central nervous system infection – it is the timepoint of fever which matters: a plea to select the best timing and optimal methods of temperature management. Brain Circ. 2017 Oct-Dec;3(4):183–5. doi:10.4103/bc.bc_31_17
Vanderschueren S, Eyckmans T, De Munter P, Knockaert D. Mortality in patients presenting with fever of unknown origin, Acta Clinica Belgica, 2014:69(1):12-16, doi:10.1179/0001551213Z.0000000005
Walter EJ, Carraretto M. The neurological and cognitive consequences of hyperthermia. Crit Care. 2016;20:199. doi:10.1186/s13054-016-1376-4
Centers for Disease Control and Prevention. Bacterial meningitis.
Barbi E, Marzuillo P, Neri E, Naviglio S, Krauss BS. Fever in children: pearls and pitfalls. Children (Basel). 2017 Sep;4(9):81. doi:10.3390/children4090081
Tesini BL, Epstein LG, Caserta MT. Clinical impact of primary infection with roseoloviruses. Curr Opin Virol. 2014;9:91–96. doi:10.1016/j.coviro.2014.09.013
National Institute of Diabetes and Digestive and Kidney Diseases. Symptoms & causes of urinary retention.

By Kristina Herndon, RN
Kristina Herndon, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.

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When Is a Fever Too High?