High-grade fevers, called hyperpyrexia, involve temperatures above 103 degrees and can be dangerous. However, when gauging a fever's danger in yourself or your child, it's more complicated than just looking at a number—especially in children.

For most people, most of the time, a fever isn't dangerous in terms of causing brain damage. This may go against what you've been taught to fear.

It helps to know what causes a fever, what the potential complications can be, and when to call your doctor or head to the emergency room. It's also important to know what's different about fevers in kids versus adults.

Why You Get Fevers

Usually, fevers are actually a good thing. They're part of the natural way the body fights off infections.

A part of the brain called the hypothalamus acts as a thermostat for your body. Most of the time, it keeps body temperatures around 98.6 degrees (37 degrees Celsius). When you get sick, though, it raises the temperature to make it hard for germs to live and multiply.

That's all well and good, unless the fever gets so high that it could start harming you.

Causes

A lot of things can cause high fevers. Some include low-grade fevers that become high-grade, such as:

Viruses
Bacteria
Fungi
Toxins
Drugs

If a fever continues unchecked, or your body doesn't respond to treatment, the fever may rise into the danger zone.

Some medical conditions that don't involve low fevers are associated with high-grade fever. Some of these are:

Intracranial hemorrhage
Thyroid storm
Serotonin syndrome
Sepsis
Kawasaki syndrome
Neuroleptic malignant syndrome
Heatstroke
Drug overdose

Symptoms and Complications

As your fever rises, it may bring on new symptoms.

Low-grade fever (pyrexia) symptoms include:

Chills
Sweats
Feeling hot
Headache
Eyes that are achy or tired
Thirst
Low appetite

Early high-grade fever (hyperpyrexia) symptoms may still include those from the list above, plus:

Extreme sweating
Dizziness and light-headedness
Muscle cramps
Fatigue
Weakness
Nausea

Persistent or worsening high-grade fever can also cause:

Contracted (small) pupils
Mild confusion
Cool, moist, pale skin
Upset stomach or vomiting
Decreased urine or inability to urinate

Longer-lasting high-grade fever or temperatures above 106.1 F can lead to:

Extreme confusion
Hallucinations
Loss of consciousness
Shallow, rapid breathing
Hot, dry, red skin
Weak, fast heartbeat
Dilated (large) pupils
Seizures

Fevers over 106.1 F (41.2 C) need medical attention to prevent serious, long-term consequences—including brain damage and death.

Brain Damage and Death

While high fevers, especially those that are prolonged, can lead to brain damage and death, this is extremely rare.

According to a 2016 study, heatstroke is the most deadly heat-related illness, killing 58% or more of the people it affects. Among survivors, most make full recoveries, but some may have long-term organ damage.

Organs that can be damaged by prolonged hyperpyrexia include:

Brain
Heart and cardiovascular system
Kidneys
Liver
Intestines

Proper treatment is the key to preventing severe complications of high-grade fevers.

Treatment

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 kill infectious agents, treating a low-grade fever can interfere with the body's efforts.

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

When fever-related symptoms are making you feel lousy or the temperature has crept up near that 103-104 F mark, an over-the-counter fever-reducing medication like Tylenol (acetaminophen) or Motrin/Advil (ibuprofen) can help you feel better.

The effects of these medications are only temporary, though. They work for 4 to 8 hours and then wear off, meaning your fever may come back and you'll need to take more. This doesn't mean something is wrong or that it didn't work.

Children and teenagers should not be given aspirin for a fever due to the risk of Reye syndrome, a rare and serious illnesses that causes brain and liver damage.

Rest and plenty of liquids can help your body weather the fever (and underlying illness), as well.

Other home remedies may help, but you need to know which ones are safe and effective, and which ones are potentially harmful.

Treatment by Temperature

For most adults and older children, symptoms are a better indicator of when it's a problem than the number on the thermometer. Still, it's good to keep an eye on how high the fever gets.

Adults and Children 4+
Temperatures	Fahrenheit	Celsius
High-Grade	103	39.4
Potentially Dangerous	104	40
Get Medical Attention	106.7 and up	41.5

With children under 3 years of age, and especially with babies, it pays to be more cautious and know what the exact temperature is.

Fever in very young children can be a sign that something serious is wrong, so it shouldn't be ignored. Your pediatrician should have an on-call number so you can get advice at any time, or, in some cases, you may want to go straight to the emergency room.

Babies and Toddlers
Temperatures by Age	Fahrenheit	Celsius	What to Do
0-3 Months	100 oral, 100.4 rectal	37.7 oral, 38 rectal	Call doctor or go to ER
3 Mo.-3 Years	102.2	39	Call doctor for advice, even after hours

Medical Care

If your fever is caused by an infection, it won't go away until the infection is gone or at least improves. Depending on the specific pathogen, this may require medical treatment.

If your fever doesn't go away or is accompanied by other symptoms that suggest illness, you should see a doctor.

If you rush to your doctor's office, urgent care, or the ER for fever treatment, you can expect medications and testing to figure out what's causing the elevated temperature.

A Word From Verywell

Adults are typically better able to determine when our symptoms are making us feel so bad that we need to seek medical treatment, but if you aren't sure, evaluate what is going on. Learn how to check your cold and flu symptoms, evaluate a fever, and know the situations when you should see a doctor for a fever.