Fever: What’s Considered Low, High, and Possibly Serious?

Fever is an elevation in the body's temperature. It's not a disease but rather a sign or response to something occurring within the body, most commonly an infection. Noninfectious conditions or scenarios like inflammatory diseases or medication reactions can also cause a fever.

What Is Considered a Normal Body Temperature?

To understand fever, it's important first to understand what a normal body temperature is.

Normal Body Temperature Ranges

There is no formal definition for a normal body temperature, although general ranges exist.

Factors affecting body temperature include age, measurement site (e.g., mouth vs. rectum vs. underarm), and time of day. Body temperature is naturally at its lowest in the early morning and highest in the late afternoon.

In adults, the average body temperature measured orally (by mouth) is 98.0 F (36.7 C), with an approximate range between 95.5 F and 99.9 F.

In most babies and children, the normal body temperature range is between 97.7 F (36.5 C) and 99.5 F (37.5 C). Their slightly higher body temperature is attributed to their greater total surface area in relation to their body weight and higher metabolisms.

What's Considered a Fever?

The hypothalamus is a small gland in the brain that controls the body's temperature, like a thermostat. Fever is an increase in body temperature that occurs when the hypothalamus's set point is shifted upwards in response to inflammation or infection.

Generally accepted ranges include the following:

• In healthy babies younger than 3 months, a fever is a rectal temperature of 100.4 F or greater.
• In children age 3 months to 36 months, a fever is a rectal temperature between 100.4 F and 102.2 F.
• In older children and adults, a fever is an oral temperature ranging from 100.0 F to 103.0 F.

Rectal temperatures should be taken for children under 4 years old, as they provide the most accurate reading. Oral temperatures are generally acceptable for children around 4 or 5 years old. Forehead thermometers may be used for all ages. Axillary and tympanic temperatures are not recommended, as they are unreliable in young children.

Low-Grade Fever Temperature Range

A low-grade fever generally indicates a body temperature that is slightly above normal. There is no formally defined range, but one research article categorized it as between 99.1 F and 100.4 F.

In children, body temperatures between 99.5 F and 101.3 F are considered by some to be elevated.

High Fever Temperature Range

A high fever indicates a body temperature that is possibly serious or dangerous.

Again, there is no formal definition, but typically, a high fever is 102.2 F or greater in babies 3 months and older and younger children.

A high fever in older children and adults is a temperature of 102.4 F or greater.

Fevers above 105.8 F are termed "hyperpyrexia" and can be seen with brain dysfunction, severe infections, or brain bleeds.

An extremely high temperature above 105.8 F sometimes represents hyperthermia, which is different from a fever. Hyperthermia can be rapidly fatal and is caused by a malfunctioning of the body's ability to manage heat coming from the environment.

Causes include:

• Excessive clothing and bedding insulation in infants
• Heatstroke
• Neuroleptic malignant syndrome (a rare side effect associated with the use of antipsychotic drugs)

How to Break a Fever at Home

Many fevers can be safely treated at home with fluids and rest, assuming no symptoms or signs of a serious illness are present.

Antipyretic Drugs

Sometimes, fever-reducing medicines—called antipyretic agents—are used to ease discomfort, especially if the fever is high or other symptoms related to the fever are present. These can include malaise (general feeling of being unwell), headache, and muscle aches.

Antipyretic agents are also often indicated in individuals with underlying heart or lung diseases, as fever can increase heart and breathing rates.

Tylenol (acetaminophen) and the nonsteroidal anti-inflammatory drug (NSAID) ibuprofen, sold under the brand names Advil or Motrin, are commonly used antipyretics.

Taking too much Tylenol can cause liver injury and damage. Likewise, too much ibuprofen and other NSAIDs can cause stomach bleeding and may need to be avoided in individuals with heart or kidney disease, high blood pressure (hypertension), or bleeding problems.

This is why it is important to read the medication label carefully when giving these medications. Information on the label can help you determine the correct dose and frequency based on your child's weight.

Also, the NSAID aspirin should never be used in kids under 19, as it's associated with Reye's syndrome., a life-threatening condition.

An antipyretic is often unnecessary in a child older than 3 months who has a rectal temperature less than 102 F (38.9 C), seems well, and is behaving like their usual selves.

Cooling Therapies

Cooling therapies—such as cooling blankets or cool-water sponge baths—can help reduce fever in adults with high fevers, but only when used concurrently with an antipyretic. This is because physical cooling of the body causes blood vessels in the skin to narrow, decreasing the body's ability to lose heat.

Cooling therapies are not recommended in babies or children with a fever-related illness.

School and Work With a Fever

To prevent the spread of infection, a child or adult should be fever-free for 24 hours without the use of fever-reducing medications) and feel and appear well before returning to school, work, or other activities.

Fever Symptoms That Warrant Medical Attention

In many instances, seeing a healthcare provider or going to the emergency room is unnecessary for a fever, as the cause is often a viral infection that needs to run its course, typically three to five days.

That said, certain factors, such as an ill-appearing child or adult, underlying medical conditions, or worrisome symptoms, warrant medical attention.

Babies and Children

According to the American Academy of Pediatrics and experts from UpToDate, seek medical attention in the following scenarios:

• A baby less than 3 months of age with a rectal reading of 100.4 F (38 C) or higher, even if the baby looks well (avoid fever medication until speaking with the healthcare provider)
• Children age 3 months to 3 years who have had a rectal reading of 100.4 F (38 C) or higher for more than one to three days or who appear ill or are fussy, unusually tired, clingy, or refuse to drink fluids
• Children aged 3 months to 3 years who have a rectal reading of 102 F (38.9 C) or higher
  
• Children of any age whose mouth/ear/rectal/forehead reading is 104 F (40 C) or higher or whose axillary (armpit) reading is 103 F (39.4 C) or higher
  
• Children of any age who have a fever and are taking steroids and/or have a chronic medical condition (e.g., cancer or sickle cell disease)
• Children of any age who have a febrile seizure
• Children of any age who have been in a sweltering environment, like an overheated car

In addition, infants and children with a fever and any of the following symptoms also require immediate medical attention:

• New skin rash
• Stiff neck
• Persistent vomiting or diarrhea
• Severe headache/sore throat/ear pain
• Signs of dehydration (e.g., fewer wet diapers, dry mouth, no tears, sunken soft spot).

Children of any age with repeating fevers (even if they only last a few hours) without other symptoms for more than seven days or who act or appear ill, even when the fever is reduced, must be seen promptly by a healthcare provider.

Adults

If you are an adult and have a fever, seek medical attention right away if you:

• Are pregnant
• Are recovering from a recent surgery or were recently hospitalized
• Are taking chemotherapy, steroids, or other drugs that weaken the immune system
• Think a tick may have bitten you
• Have a chronic health condition like diabetes, heart disease, or cancer
• Have recently returned from a trip abroad, namely from Africa, Asia, Latin America, or the Middle East
• Have fevers that recur or persist

Go to your nearest emergency room if your fever is accompanied by one or more of the following symptoms:

• Skin rash
• Problems breathing
• Severe headache or neck pain/stiffness
• Seizure
• Feeling confused
• Severe or persistent vomiting or diarrhea
• Pain in the stomach, back, or sides
  

Causes of Fever

There are multiple potential causes of fever.

Viral Infections are the most common cause. Examples include:

• The common cold
• The flu (influenza)
• COVID-19
• Acute bronchitis
• Viral stomach flu (e.g., rotavirus in young children)

Other causes include bacterial infections such as:

• Urinary tract infections (UTIs)
• Bacterial gastroenteritis (e.g., Salmonella)
• Bacterial otitis media (middle ear infection)
• Skin infections (e.g., cellulitis)
• Pneumonia (lung infection)

Sometimes, fever is caused by a non-infectious condition such as:

• Autoimmune diseases (e.g., lupus or rheumatoid arthritis)
• Cancer
• Reaction to a medication, vaccine, or blood transfusion
• Acute pancreatitis (inflammation of the pancreas)
• Acute cholecystitis (inflammation of the gallbladder)
  

Summary

Fever is an elevation of the body's temperature and occurs when the set-point of the brain's thermostat shifts upwards.

Fever is a response to something in the body, usually a viral infection like the flu or COVID-19. Bacterial infections, such as urinary tract or skin infections, can also trigger fevers, as can noninfectious causes like autoimmune diseases or drug reactions.

No formal or standard definition exists for fever, as it can vary depending on age, time of day, and where it's measured on the body (e.g., mouth, rectal, ear, underarm, or forehead).

Most cases of fever are due to viral infections. They can be managed at home with fluids, rest, and, sometimes, a fever-reducing medication like Tylenol or ibuprofen (under the guidance of a healthcare provider)

That said, certain factors warrant medical attention—for example, a baby less than three months of age with a rectal reading of 100.4 F (38 C) or higher or a young child with a rectal reading of 102 F (38.9 C) or higher.

Likewise, the presence of worrisome symptoms, in addition to the fever, like trouble breathing or signs of dehydration, requires immediate medical attention. While many fevers can be managed safely at home, contact a healthcare provider if you have questions or concerns or if your gut instinct is kicking in.