Fever: Symptoms, Causes & When to Seek Care

What Is Fever and Why Does It Happen?

Fever is an elevated body temperature above 38°C (100.4°F) that occurs as part of your immune system's response to infections and other conditions. It is not a disease itself but a symptom indicating your body is fighting something, most commonly viral or bacterial infections.

Fever represents one of the most fundamental and evolutionarily conserved defense mechanisms in the human body. When pathogens such as viruses, bacteria, or other infectious agents enter your body, your immune system initiates a complex cascade of responses designed to eliminate the threat. One of the key components of this response is the elevation of core body temperature, which creates an environment less favorable for pathogen reproduction.

The process begins when your immune cells detect foreign invaders and release signaling molecules called pyrogens. These pyrogens travel to the hypothalamus, the region of your brain that acts as your body's thermostat. In response, the hypothalamus raises the set point for body temperature, triggering mechanisms such as shivering (to generate heat) and vasoconstriction (to conserve heat). This coordinated response results in the fever you experience.

Understanding that fever serves a protective function is important. While the elevated temperature can feel uncomfortable, it actually helps your immune system work more efficiently. Higher temperatures increase the activity of immune cells, accelerate the production of antibodies, and directly inhibit the growth of many pathogens that thrive at normal body temperature. This is why healthcare providers generally do not recommend aggressive treatment to eliminate all fevers, especially low-grade ones.

What Causes Fever?

The most common cause of fever is infection, with viral infections accounting for the majority of cases. When you develop a cold, the flu, or other viral illness, fever is typically one of the first symptoms to appear. Your body recognizes the viral particles as foreign and mounts an immune response that includes raising your temperature.

Bacterial infections also frequently cause fever. Conditions such as urinary tract infections, pneumonia, strep throat, and skin infections trigger similar immune responses. Bacterial infections often cause higher and more prolonged fevers than viral infections, though this is not a reliable way to distinguish between the two.

• Viral infections: Common cold, influenza, COVID-19, gastroenteritis, and many childhood illnesses
• Bacterial infections: Urinary tract infections, pneumonia, skin infections, strep throat
• Inflammatory conditions: Rheumatoid arthritis, inflammatory bowel disease, lupus
• Medications: Some antibiotics, antihistamines, and seizure medications can cause drug-induced fever
• Heat-related illness: Heat exhaustion and heat stroke
• Malignancies: Certain cancers, particularly lymphomas and leukemias
• Post-vaccination response: Normal immune response to vaccines

What Are the Signs and Symptoms of Fever?

The primary sign of fever is an elevated body temperature above 38°C (100.4°F). However, fever is typically accompanied by other symptoms including chills and shivering when temperature is rising, sweating when temperature is falling, general weakness, headache, muscle aches, and loss of appetite.

Recognizing fever involves more than just measuring temperature. Many people can sense when they have a fever based on how they feel overall. The subjective experience of fever often includes a general sense of being unwell that goes beyond simply feeling warm. Understanding the full spectrum of fever symptoms helps you assess your condition and determine when additional care might be needed.

The symptoms you experience during fever reflect the underlying physiological changes occurring in your body. As your hypothalamus raises your temperature set point, your body must generate and conserve heat to reach this new target. This process produces the characteristic chills and shivering associated with rising fever. Conversely, when your body begins to lower the temperature (either naturally or with medication), you may experience profuse sweating as heat is released.

Beyond the temperature-related symptoms, fever often causes systemic effects that affect your entire body. These include fatigue and weakness, which encourage rest and allow your body to direct energy toward fighting the infection. Headache and muscle aches are common companions to fever, resulting from the inflammatory processes occurring throughout your body.

What Temperature Counts as Fever?

Normal body temperature varies somewhat from person to person and throughout the day. For most adults, normal body temperature ranges from 36°C to 37.8°C (96.8°F to 100°F). A temperature of 38°C (100.4°F) or higher is generally considered a fever. However, the absolute number matters less than the overall clinical picture - how you feel and whether your temperature is significantly elevated compared to your personal baseline.

Children typically run slightly warmer than adults, so temperature thresholds for fever may differ. The method of measurement also affects the reading: rectal temperatures are typically about 0.5°C (1°F) higher than oral temperatures, while axillary (armpit) temperatures are often 0.5°C lower than oral readings. Ear (tympanic) thermometers provide readings closest to core body temperature when used correctly.

How to Measure Body Temperature Correctly

Accurate temperature measurement is important for monitoring fever and communicating with healthcare providers. Several methods are available, each with advantages and considerations. Choosing the right method and technique ensures you get reliable readings.

For the most accurate results, rest for at least 30 minutes before taking your temperature. Avoid measuring immediately after eating, drinking, exercising, or bathing, as these activities can temporarily affect readings. The best time for a baseline measurement is typically in the morning before getting out of bed.

• Ear (tympanic) thermometers: Quick and convenient, providing readings in seconds. Pull the ear back slightly to straighten the ear canal for accurate results
• Oral thermometers: Place under the tongue with mouth closed for the specified time. Do not use immediately after consuming hot or cold beverages
• Rectal thermometers: Most accurate method, particularly for infants. Insert about 2.5 cm (1 inch) and hold in place
• Forehead (temporal artery) thermometers: Non-invasive and fast, but may be less accurate
• Axillary (armpit) thermometers: Least accurate but non-invasive. Add about 0.5°C to reading for better estimate

When Should You See a Doctor for Fever?

Seek medical care if fever lasts more than 4 days, persists more than 2 days after starting antibiotics, occurs after surgery or medical procedures, follows travel to tropical regions, or is accompanied by severe symptoms such as stiff neck, confusion, difficulty breathing, or severe headache. People with compromised immune systems or serious underlying conditions should seek care sooner.

Most fevers, particularly those accompanying common viral infections, resolve on their own within a few days and do not require medical intervention. However, certain situations warrant professional evaluation. Understanding these warning signs helps you make informed decisions about when to seek care and when home management is appropriate.

The duration of fever provides important information. A fever lasting more than four days suggests that your body may need additional support to fight the underlying condition. Similarly, if you have started antibiotic treatment for a bacterial infection but your fever persists beyond 48 hours, the antibiotic may not be effective against the specific bacteria, or there may be complications requiring attention.

Context matters significantly when evaluating fever. Fever following surgery or medical procedures may indicate infection at the surgical site or elsewhere and requires prompt evaluation. Fever after travel to tropical or subtropical regions raises concern for malaria, dengue fever, and other travel-related infections that require specific testing and treatment.

Special Considerations for High-Risk Groups

Certain individuals face higher risks from fever and should seek medical attention more promptly. People with compromised immune systems - whether from conditions like HIV/AIDS, medications such as chemotherapy or immunosuppressants, or organ transplants - cannot fight infections as effectively. Even mild fevers in these individuals may indicate serious infections requiring immediate treatment.

Those with chronic medical conditions such as heart disease, lung disease, diabetes, or kidney disease should also have lower thresholds for seeking evaluation. Fever places additional stress on the cardiovascular system and can worsen underlying conditions. Additionally, elderly adults may not mount the typical fever response, meaning even lower-grade fevers can indicate significant illness.

How Can You Treat Fever at Home?

Home treatment for fever focuses on staying comfortable and supporting your body's recovery. Key measures include drinking plenty of fluids to prevent dehydration, resting without necessarily staying in bed continuously, keeping your environment cool but comfortable, and using over-the-counter medications like paracetamol or ibuprofen if needed to reduce discomfort.

Managing fever at home centers on supportive care that helps your body recover while minimizing discomfort. Remember that fever itself is helping your immune system fight infection, so the goal is not necessarily to eliminate the fever entirely but to keep yourself comfortable and prevent complications like dehydration.

Hydration stands as the single most important aspect of fever management. When you have fever, your body loses more fluid than usual through sweating, rapid breathing, and increased metabolic activity. Dehydration can develop quickly, especially if you also have symptoms like vomiting or diarrhea. Water is excellent, but broths, diluted fruit juices, and electrolyte solutions also help replace lost fluids and minerals.

Rest supports your immune system's efforts but does not require staying in bed continuously. Moving around periodically helps prevent complications from prolonged immobility and can actually make you feel better. However, strenuous activity and exercise should be avoided entirely until your fever has resolved - exercising while febrile puts additional strain on your heart and can worsen your condition.

Managing Comfort During Fever

The environment you rest in affects your comfort during fever. Keep your room cool but not cold - around 18-20°C (64-68°F) is ideal for most people. Good air circulation helps, and wearing light, breathable clothing allows your body to regulate temperature more effectively.

The characteristic pattern of fever involves feeling cold with chills as temperature rises, then hot and sweaty as it falls. Adapt your clothing and covers accordingly: add layers when experiencing chills, and remove them when you begin sweating. This responsiveness to your body's signals helps maintain comfort throughout the fever cycle.

Over-the-Counter Fever Medications

Antipyretic (fever-reducing) medications can provide relief when fever causes significant discomfort. The two main options available without prescription are paracetamol (acetaminophen) and non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen. Both effectively reduce fever and provide pain relief for associated symptoms like headache and body aches.

Paracetamol (acetaminophen) works primarily by affecting the temperature-regulating center in the brain. It is generally well-tolerated and safe when used as directed, making it suitable for most adults. However, it is important not to exceed the recommended daily dose, as overdose can cause serious liver damage.

NSAIDs like ibuprofen not only reduce fever but also decrease inflammation, which can be helpful when infection involves significant inflammatory symptoms. However, NSAIDs are not suitable for everyone - they should be used cautiously or avoided by people with stomach problems, kidney disease, heart conditions, or those taking blood thinners.

Can Fever Be Prevented?

While fever itself cannot be directly prevented (it is a natural immune response), you can reduce your risk of infections that cause fever through good hygiene practices, staying current with vaccinations, maintaining a healthy lifestyle, and avoiding contact with sick individuals when possible.

Since fever is a symptom rather than a disease, prevention efforts focus on reducing your risk of the infections and conditions that trigger febrile responses. Many of these measures align with general health practices that protect against a wide range of illnesses.

Hand hygiene remains one of the most effective ways to prevent infection transmission. Washing hands thoroughly with soap and water for at least 20 seconds - especially before eating, after using the bathroom, and after being in public spaces - removes pathogens before they can enter your body. When soap and water are not available, alcohol-based hand sanitizers provide an alternative.

Respiratory hygiene also plays a crucial role in preventing infection spread. Covering coughs and sneezes with a tissue or your elbow, avoiding touching your face, and maintaining distance from people who are visibly ill all reduce transmission risk. During periods of high respiratory illness activity, wearing masks in crowded indoor spaces provides additional protection.

Vaccination and Immune Health

Vaccinations provide powerful protection against many infections that cause fever. Staying current with recommended immunizations - including annual influenza vaccines, COVID-19 boosters, and any other vaccines recommended based on your age, health status, and circumstances - significantly reduces your risk of these preventable diseases.

Supporting your overall immune health through lifestyle factors also contributes to infection resistance. Regular physical activity (when healthy), adequate sleep, a balanced diet rich in fruits and vegetables, stress management, and avoiding smoking all support optimal immune function. While these measures cannot guarantee you will never develop fever, they give your body the best chance of fighting off infections before they take hold.

Are There Complications from Fever?

Most fevers in healthy adults resolve without complications. However, very high fevers (above 40°C/104°F) can be dangerous, and prolonged fever increases the risk of dehydration. Febrile seizures can occur in young children but are generally not harmful. The underlying condition causing fever may have its own complications that require treatment.

For the vast majority of healthy adults, fever is a temporary discomfort that resolves without lasting effects. Your body is well-equipped to handle the temperature elevations associated with fighting common infections. Understanding potential complications helps you recognize situations requiring additional attention without causing unnecessary worry about typical fever episodes.

Dehydration represents the most common complication of fever and is largely preventable with adequate fluid intake. The combination of increased fluid loss through sweating and reduced appetite during illness can quickly lead to dehydration if fluids are not actively replaced. Signs of dehydration include dark urine, decreased urination, dry mouth, dizziness upon standing, and extreme thirst.

Very high fevers - generally those exceeding 40°C (104°F) - can potentially cause problems if sustained. At extreme temperatures, proteins in the body can begin to denature, affecting cellular function. However, the body has protective mechanisms that prevent most fever-causing conditions from raising temperature to dangerous levels. Seek medical attention for temperatures in this range.

Understanding Febrile Seizures

Febrile seizures occur in some young children (typically between 6 months and 5 years of age) during fever. While witnessing a seizure is frightening for parents, febrile seizures are generally not harmful and do not indicate epilepsy or cause brain damage. Most febrile seizures last less than five minutes and stop on their own.

If a febrile seizure occurs, place the child on a safe surface on their side, clear the area of any objects that could cause injury, and do not place anything in their mouth. Time the seizure if possible. Seek immediate medical attention if the seizure lasts more than five minutes, if the child has difficulty breathing afterward, or if this is the first febrile seizure.