What causes a high-grade fever of 103°F or higher in adults. How to properly measure body temperature. When should you worry about a fever in adults. What are the potential underlying causes of unexplained fevers. How to treat high fevers at home and when to seek immediate medical care.

Understanding Fever: Definition, Causes, and Significance

Fever is a common physiological response that occurs when the body’s temperature rises above its normal range. But what exactly constitutes a fever, and why does our body initiate this response?

A fever is defined as a body temperature of 100.4°F (38°C) or higher. It’s important to note that normal body temperature can vary slightly from person to person and throughout the day. While 98.6°F (37°C) has long been considered the standard “normal” temperature, recent research suggests that average body temperatures may be slightly lower, ranging from 97°F to 99°F (36.1°C to 37.2°C).

Why do we get fevers? Fevers are typically the body’s natural defense mechanism against infections. When pathogens like viruses or bacteria invade our system, the hypothalamus – our brain’s temperature control center – raises the body’s internal thermostat. This increase in temperature creates an environment that’s less hospitable for these invading microorganisms, while simultaneously boosting our immune response.

Common Causes of Fever in Adults

• Viral infections (e.g., influenza, COVID-19, common cold)
• Bacterial infections (e.g., urinary tract infections, pneumonia)
• Fungal infections
• Inflammatory conditions
• Certain medications
• Heat exhaustion or heatstroke
• Some cancers
• Autoimmune disorders

Interestingly, it’s possible for adults to develop a fever without experiencing any other noticeable symptoms. This phenomenon, known as a fever of unknown origin (FUO), can be particularly perplexing for both patients and healthcare providers.

High-Grade Fevers: When Temperature Spikes to 103°F or Higher

While any elevation in body temperature above 100.4°F is considered a fever, temperatures of 103°F (39.4°C) or higher in adults are classified as high-grade fevers. These elevated temperatures can be concerning and often warrant closer attention.

High-grade fevers can be caused by various factors, including:

• Severe viral or bacterial infections
• Sepsis
• Meningitis
• Endocarditis
• Certain tropical diseases
• Some autoimmune conditions

Is a high-grade fever always a cause for immediate concern? Not necessarily, but it does require careful monitoring. In many cases, even high fevers can be the body’s appropriate response to fighting off an infection. However, persistent high-grade fevers or those accompanied by other concerning symptoms should prompt medical evaluation.

Measuring Fever Accurately

To ensure accurate temperature readings, it’s crucial to use the right technique. For adults, oral temperature measurement is generally considered the most reliable method. Here are some tips for taking an accurate oral temperature:

1. Wait at least 30 minutes after eating or drinking hot or cold substances
2. Keep the thermometer under the tongue for the recommended time (usually about 1 minute for digital thermometers)
3. Avoid taking temperature immediately after physical activity or a hot shower
4. For consistency, try to measure at the same time of day, as body temperature naturally fluctuates throughout the day

Fever in the Context of COVID-19: What You Need to Know

In the era of COVID-19, fever has taken on new significance as one of the primary symptoms of the disease. However, it’s important to remember that not all fevers are indicative of COVID-19, and not all cases of COVID-19 present with fever.

Can fully vaccinated individuals develop a fever due to COVID-19? While breakthrough infections in vaccinated individuals are possible, they tend to be less severe. Fever, if present, is often milder compared to infections in unvaccinated individuals. However, any unexplained fever, especially if accompanied by other symptoms like cough, fatigue, or loss of taste or smell, should be evaluated by a healthcare provider.

If you develop a fever and are concerned about COVID-19, it’s advisable to isolate yourself and contact your healthcare provider for guidance on testing and further management.

When to Seek Medical Attention for a Fever

While fevers are often benign and self-limiting, there are circumstances when medical attention is warranted. Here are some guidelines for when to seek medical care for a fever:

• Temperature of 103°F (39.4°C) or higher that doesn’t respond to over-the-counter fever reducers
• Fever persisting for more than three days
• Fever accompanied by severe headache, neck stiffness, or confusion
• Presence of other concerning symptoms such as severe abdominal pain, difficulty breathing, or chest pain
• Recent travel to areas with endemic infectious diseases
• Underlying health conditions that increase risk (e.g., diabetes, heart disease, compromised immune system)

For individuals undergoing chemotherapy or with severely compromised immune systems, any fever lasting more than an hour should prompt immediate medical attention due to the risk of life-threatening infections.

Home Management of Fevers: Tips and Techniques

While high-grade fevers often require medical evaluation, many fevers can be managed effectively at home. Here are some strategies for managing fever and promoting comfort:

1. Stay hydrated: Fever can lead to increased fluid loss through sweating. Drink plenty of water, clear broths, or electrolyte solutions.
2. Rest: Allow your body to conserve energy for fighting the infection.
3. Use over-the-counter fever reducers: Acetaminophen or ibuprofen can help lower fever and alleviate discomfort. Always follow recommended dosages.
4. Cool compresses: Applying a cool, damp cloth to the forehead or wrists can provide relief.
5. Dress appropriately: Wear light, breathable clothing and use lightweight bedding to prevent overheating.
6. Monitor temperature: Keep track of fever trends to share with your healthcare provider if needed.

Remember, the goal isn’t necessarily to eliminate the fever entirely, as it plays a role in fighting infections. Instead, focus on managing discomfort and preventing dehydration.

Fever in Special Populations: Considerations for Older Adults and Pregnant Women

Certain groups may require special consideration when it comes to fever management and evaluation. Two such populations are older adults and pregnant women.

Fever in Older Adults

Older adults, particularly those over 65, may not mount the same fever response as younger individuals. In fact, serious infections in older adults might not cause a significant fever or may even lead to a drop in body temperature. This altered response can make infections harder to detect and potentially more dangerous.

For older adults, any change in normal temperature patterns or the presence of other symptoms like confusion, weakness, or loss of appetite should prompt medical evaluation, even if a high fever isn’t present.

Fever During Pregnancy

Pregnant women should be particularly cautious about fevers, especially in the first trimester. High body temperatures during early pregnancy have been associated with an increased risk of certain birth defects.

If you’re pregnant and develop a fever:

• Contact your healthcare provider promptly
• Use acetaminophen to reduce fever, as it’s generally considered safe during pregnancy (always consult your doctor first)
• Stay well-hydrated
• Monitor fetal movement and report any changes to your healthcare provider

Beyond Infection: Other Potential Causes of Unexplained Fevers

While infections are the most common cause of fevers, there are other potential culprits that shouldn’t be overlooked, especially in cases of persistent or recurrent fevers.

Non-Infectious Causes of Fever

• Autoimmune disorders: Conditions like rheumatoid arthritis, lupus, or inflammatory bowel disease can cause recurrent fevers.
• Medications: Some drugs, including antibiotics and seizure medications, can cause drug fevers.
• Cancers: Certain cancers, particularly lymphomas and leukemias, can present with fever as an early symptom.
• Hormonal disorders: Conditions affecting the thyroid or adrenal glands can sometimes cause temperature dysregulation.
• Blood clots: Deep vein thrombosis or pulmonary embolism can be associated with fever.
• Metabolic disorders: Conditions like gout or porphyria can occasionally present with fever.

When fever persists without an apparent cause, your healthcare provider may conduct a thorough workup to investigate these less common causes. This might include blood tests, imaging studies, and in some cases, specialized tests to rule out specific conditions.

The Evolution of “Normal” Body Temperature: Debunking the 98.6°F Myth

For centuries, 98.6°F (37°C) has been considered the gold standard for normal body temperature. This figure originated from research conducted by German physician Carl Reinhold August Wunderlich in the 19th century. However, recent studies have challenged this long-held belief.

Modern research suggests that average body temperatures have been declining over the past two centuries. A 2020 study published in eLife found that the average body temperature of Americans has dropped by about 0.05°F per decade since the early 19th century.

Why might body temperatures be falling? Several theories have been proposed:

• Reduced rates of chronic inflammation due to improved hygiene and healthcare
• Lower metabolic rates associated with increased body mass and reduced physical activity
• Environmental factors such as climate-controlled living spaces
• More accurate thermometers compared to those used in the 19th century

What does this mean for fever detection? It underscores the importance of understanding your own baseline temperature and recognizing that “normal” can vary from person to person. A temperature that constitutes a fever for one individual might be within the normal range for another.

In light of these findings, many healthcare providers now consider a wider range of temperatures as “normal,” typically between 97°F and 99°F (36.1°C to 37.2°C). However, the definition of fever (100.4°F or higher) remains unchanged, as it’s based on the temperature at which the body’s immune response is typically activated.

Understanding these nuances can help individuals make more informed decisions about when to seek medical care for elevated temperatures. It also highlights the importance of considering other symptoms and overall health status, rather than relying solely on a specific temperature threshold to define illness.

Fever & Temperature In Adults: When To Worry

Karen Justiniano, DO, MS, Stamford Health Medical Group

Looking for a Primary Care Physician (PCP)?

Let us help you find the right doctor for your needs.

Explore Your Options

A friend of mine whose father is 70 years old told me her story. It went something like this:

“My father never gets fevers. Out of nowhere the other day, he developed a 103 temperature with no other symptoms. He is also fully vaccinated against COVID-19. My mother brought him to the emergency room. Did she do the right thing, or would he likely have been fine if he stayed home and let the fever run its course?”

As a physician who cares for a variety of people with a variety of conditions, I have to say of course she did the right thing! When in doubt or when concerned, seeking medical attention is the smart thing to do.

But the longer answer is a little more complicated than that. It all starts with breaking down the purpose of a fever, and when you should truly worry. Please note—fevers in infants or young children are sometimes a different story—what you are about to read relates to adults only.

What is a fever and why do we get them, and sometimes with no other symptoms?

Fever is an elevated body temperature which means your temperature is 100.4 degree F or higher. A fever is the body’s way of fighting an infection by naturally raising your body’s temperature to “kill the germ.” The medical community likes to say that fever is a “good thing,” in most cases.

And yes, it’s completely possible for adults to develop a fever with no other symptoms, and for doctors to never truly find the cause. Viral Infections can commonly cause fevers, and such infections include COVID-19, cold or the flu, airway infection like bronchitis, or the classic stomach bug.

That’s why you often hear the word “viral” as an explanation because these types of infections often go away on their own after a few days.

What is considered a high fever in adults?

A high-grade fever in adults is 103 degrees F or higher.

Is it possible that a fever could be a sign of a “breakthrough case” of COVID-19 even if the person is fully vaccinated?

Yes, but not highly likely. Breakthrough cases of COVID-19 in fully vaccinated people have been reported, but symptoms tend to be milder than in non-vaccinated people, including fever.

First, make sure you are taking your temperature the right way. Taking your temperature by mouth is the most accurate method, and wait at least 30 minutes after you eat or drink anything hot or cold. Get medical attention for your fever if:

• Your temperature is high and has not gone down after taking Tylenol or Advil
• Your temperature lasts several days or keeps coming back
• You live in an area where people have COVID-19
• You have serious health conditions like diabetes, heart disease, cancer, lupus, or sickle cell anemia. (Non-infections like these can cause fever as well.)

You should also seek medial attention if you have a fever and you:

• Are pregnant
• Recently returned from travel overseas
• Get infections often
• Are on chemotherapy, in which case you should seek medical attention immediately if the fever lasts for more than one hour.
• Have recently been bitten by a tick

Is 98.6 still considered “normal” body temperature?

Yes and no. You’ve probably always heard that the average body temperature is 98.6, but the truth is there’s a much wider range—from about 97 to 99 degrees F.

So why is 98.6 suddenly a myth after all these centuries? Research suggests body temperatures are falling overall. Doctors have a few ideas about why this is, including lower metabolic rates, lower rates of infection and more advanced thermometers.

A couple of helpful reminders:

1. As the saying goes, “Don’t fear the fever.” A fever is your body’s first line of defense in fighting off an infection.
2. If you are not fully vaccinated against COVID-19, and have developed a high-grade fever, please call your doctor right away.

High Temperature Causes and Treatments

Written by Rick Ansorge

• What Is a Fever?
• Fever Symptoms
• Fever Causes
• Fever Diagnosis
• Fever Treatments

A fever is a body temperature that’s higher than is considered normal. It’s also called a high temperature, hyperthermia, or pyrexia, and it’s usually a sign that your body is working to keep you healthy from an infection. Normal body temperatures are different for everyone, but they lie within the range of 97 to 99. A temperature of 100.4 or higher is considered a fever.

A part of your brain called the hypothalamus controls your body temperature. In response to an infection, illness, or some other cause, the hypothalamus may reset the body to a higher temperature. So when a fever comes on, it’s a sign that something is going on in your body.

Fevers themselves generally aren’t dangerous, but you should check in with your doctor if:

• An adult’s temperature is 103 or higher
• A very young infant (under 3 months) has a rectal temperature 100.4 or higher (call your doctor or go to an emergency room immediately)
• A 3-6-month-old has a higher than normal rectal temperature and is also irritable or sleepy (call your doctor right away)
• A 3-6-month-old has a 102 or higher rectal temperature
• A 6-24-month-old has a fever higher than 102 for more than a day or with other symptoms such as a cough or diarrhea
• A child older than 2 has a fever that comes with rash, real discomfort, irritability, listlessness, headache, stiff neck, or repeated diarrhea or vomiting
• An infant or child has a seizure
• Any temperature over 104 in a child, which could cause a seizure
• Any fever that starts after someone has been in hot temperatures, which could be a sign of heat stroke
• The fever doesn’t go down after taking over-the-counter medications such as ibuprofen in the appropriate doses
• You’ve been in contact with someone who has COVID-19

Fevers are signs of some sort of illness or infection. When you have, you may also notice these symptoms:

• Chills or shivering
• Sweating
• Headache
• Feeling weak
• Being irritable
• Losing your appetite
• Being dehydrated

A fever can be a sign of several health conditions, which may or may not need medical treatment.

The most common causes of fever are infections such as colds and stomach bugs (gastroenteritis). Other causes include:

• Infections of the ear, lung, skin, throat, bladder, or kidney
• Heat exhaustion
• COVID-19
• Sunburn
• Conditions that cause inflammation, such as rheumatoid arthritis
• Side effects of medications
• Vaccines and immunizations
• Blood clots
• Autoimmune conditions such as lupus and inflammatory bowel disease (IBS)
• Cancer
• Hormone disorders such as hyperthyroidism
• Illegal drugs such as amphetamines and cocaine
• Teething in babies can cause a mild, low-grade fever (not over 101 degrees)

Although a fever is easy to measure with a thermometer, finding its cause can be hard. Besides a physical exam, your doctor will ask about symptoms and conditions, medications, and if you’ve recently traveled to areas with infections or have other infection risks. A malaria infection, for example, may cause a fever that typically comes back. Some areas of the U.S. are hot spots for infections such as Lyme disease and Rocky Mountain spotted fever.

Your doctor may ask if you have been around someone with COVID-19 or have any other symptoms of COVID-19.

Sometimes, you may have a “fever of unknown origin.” In such cases, the cause could be an unusual or not obvious condition such as a chronic infection, a connective tissue disorder, cancer, or another problem.

Fever is usually associated with physical discomfort, and most people feel better when a fever is treated. But depending on your age, physical condition, and the underlying cause of your fever, you may or may not require medical treatment for the fever alone. Many experts believe that fever is a natural bodily defense against infection. There are also many non-infectious causes of fever.

Treatments vary depending on the cause of the fever. For example, antibiotics would be used for a bacterial infection such as strep throat.

The most common treatments for fever include over-the-counter drugs such as acetaminophen and nonsteroidal anti-inflammatory drugs such as ibuprofen and naproxen. Children and teens should not take aspirin because it’s linked to a condition called Reye’s syndrome.

Stay comfortable by: 

• Drinking a lot of clear liquids such as water, broth, and juices or a rehydration drink.
• Taking a lukewarm bath.
• Resting.
• Keeping yourself cool with lightweight clothing and bed coverings.

Top Picks

Fever – Humanitas

Fever is usually a signal that something is wrong in the body. In an adult, fever is accompanied by malaise, however, as a rule, a temperature increase of no more than 103 F (39.4 C) is considered harmless. In children of the first days of life and infants, even a slight increase in temperature may indicate the presence of a serious infection.

However, the severity of the fever does not always indicate the severity of the underlying condition. A mild illness may cause a fever with a significant increase in temperature, and a severe illness may be accompanied by a slight increase in temperature.

The fever usually resolves within a few days. A number of over-the-counter drugs can lower a fever, but in some cases it is better not to lower it. An increase in body temperature greatly contributes to the body’s fight against a number of infectious diseases.

Symptoms

Fever is an increase in temperature above normal values. The temperature normal for an individual may be slightly above or below the average normal temperature 98. 6 F (37 C).

Fever, depending on the cause, may be accompanied by signs and symptoms such as:

• Sweating
• Trembling
• Headache
• Muscle pain
• Loss of appetite
• Dehydration
• General weakness

Fever 103 F (39.4 C) to 106 F (41.1 C) may cause:

• Hallucinations
• Confusion
• Irritability
• Convulsions
• Dehydration

Conditions requiring medical attention

Fever in itself is not a cause for concern and a reason to seek medical attention. However, there are times when, if you have a fever, you need to see a doctor yourself or take your child to the doctor.

Temperature measurement

Several types of thermometers are suitable for taking your own and your baby’s temperature, including oral, rectal, and ear (tympanic) thermometers.

Although not the most accurate method, an oral thermometer can be used to measure armpit temperature (axillary temperature).

• Place the thermometer in the armpit and cross your arms over your chest.
• Wait four or five minutes. Axillary temperature is slightly lower than oral.
• The doctor who came on call must be informed of the actual readings of the thermometer and name the place where the temperature was taken.

A rectal thermometer is suitable for measuring temperature in young children:

• To do this, grease the bulb of the thermometer with petroleum jelly.
• Place the baby on the stomach.
• Carefully insert the bulb of the thermometer half a centimeter into the child’s rectum.
• Hold the thermometer and child still for three minutes.
• Never let go of the thermometer while it is in the child’s body. If the child wriggles, the thermometer may go deeper and cause damage.

Infants

Unlike adults, unexplained fever in children is a serious cause for concern. If your child’s temperature rises to 101 F (38. 3 C) or more, contact your pediatrician. You should also call a doctor in the following cases:

• Fever in a child under 3 months old.
• Refusal of the child to eat and drink.
• Fever and unexplained irritability: the child cries a lot, for example when changing a diaper or at the slightest movement.
• Lethargy and lack of reaction in a child with fever. In infants and children under 2 years of age, these signs may be signs of meningitis, an infection and inflammation of the membranes and fluid surrounding the brain and spinal cord. If meningitis is suspected, the child should be seen by a doctor immediately.
• The body temperature of the newborn is below normal – below 97 F (36.1 C). In the youngest children, during illness, body temperature is difficult to regulate: it may fall rather than rise.

Children

If a child has a fever, but still reacts to others: makes eye contact, responds to facial expressions and voice, then there is probably nothing to worry about.

In the following cases, the child should be taken to the doctor:

• Lethargy or irritability, frequent vomiting, severe headaches or abdominal pain, or any other symptoms that cause severe discomfort.
• Fever after being in a hot car. Seek immediate medical attention if:
• Fever persists for more than one day (in children under 2 years of age) or more than three days (in children 2 years of age and older)

In special cases, for example, if the child has diseases associated with disorders of the immune system or any pre-existing diseases. Doctors also advise caution if a child has recently been prescribed a new drug.

Adults

Seek medical attention if:

• Temperature rises above 103 F (39.4 C)
• Persistence of fever for more than three days

In addition, seek immediate medical attention if any of the following signs or symptoms accompany a fever:

• Severe headache
• Severe swelling of the throat
• Unusual skin rash, especially if it progresses rapidly
• Unusual sensitivity to bright light
• Stiff neck and pain when bending head forward
• Confusion
• Prolonged vomiting
• Labored breathing or chest pain
• Extreme lethargy or excitability
• Abdominal pain or pain when urinating
• Other unexplained signs and symptoms

Complications

Fever may be accompanied by the following complications:

• Severe dehydration
• Hallucinations
• Fever-induced convulsions (febrile convulsions) in a small number of children aged 6 months to 5 years
• Febrile convulsions

Febrile seizures are usually accompanied by loss of consciousness and convulsions of the limbs, involving muscle groups on both sides of the body. Although febrile seizures are a cause of concern for parents, in most cases they do not have any long-term consequences.

If ​​a febrile seizure occurs:

• Place the child on its side or stomach on the floor or other flat surface
• Remove all sharp objects within reach of the child
• Rid the child of tight clothing
• Hold the child to avoid injury.

Do not put anything in the child’s mouth or try to stop seizures
In most cases, seizures go away on their own. After the seizures stop, it is necessary to show the child to the doctor as soon as possible to determine the cause of the fever.

If convulsions persist for more than 10 minutes, seek emergency medical attention.

high temperature, what to do?

High temperature is an increase in body temperature above normal values. Normal body temperature in the armpit is 36.0-36.9ºС, and in the morning it is 0.3-0.5ºС lower than in the evening. In the oral cavity and rectum, the temperature is usually 0.5-1 ºС higher than in the armpit, but does not exceed 37.5 ºС.

The causes of fever are manifold. The most common of these are infectious diseases. Microorganisms, their toxins and waste products affect the thermoregulatory center in the brain, causing an increase in body temperature.

There are several types of fever. According to the degree of temperature rise, subfebrile is distinguished – not higher than 37.5ºС; and febrile fever. Everyone is familiar with the manifestations of fever: muscle aches, headache, pain in the eyes, weakness, chills. Chills are nothing more than a physiological way to increase body temperature. When muscles contract, they increase heat production, and body temperature rises.

An increase in body temperature during an infectious disease does not occur by chance. Fever has important physiological significance. First, many bacteria die at high temperatures or lose their ability to reproduce. In addition, with an increase in body temperature, the body’s defense mechanisms to fight infection increase. Therefore, if the fever is benign and there are no other symptoms, drug treatment is usually not required – plentiful warm drinking and rest are recommended.

However, there are negative consequences of fever. In addition to subjectively unpleasant sensations, fever leads to increased fluid loss and excessive energy expenditure, which can be dangerous in people suffering from cardiovascular and other chronic diseases. High fever is especially dangerous for children who have an increased tendency to convulsions.

When should the temperature be lowered?

• For severe discomfort.
• Sleep disorders.
• When the temperature rises above 38.5°C.

Tips for lowering the temperature:

The room should not be too hot and should be ventilated regularly. Do not rub the patient’s skin with alcohol – this can increase the chills.