Fever: Symptoms, Causes & When to Seek Care

What Causes Fever?

Fever is most commonly caused by infections, including viral infections (common cold, flu, COVID-19) and bacterial infections (urinary tract infections, pneumonia, strep throat). The body raises its temperature as a defense mechanism to create an unfavorable environment for pathogens and to enhance immune system function.

When your body detects the presence of foreign invaders such as viruses, bacteria, or other pathogens, your immune system springs into action. One of its most effective responses is to raise your body temperature through a complex process involving the hypothalamus, the brain's temperature-regulating center. This elevated temperature serves multiple purposes: it makes it harder for many pathogens to reproduce, enhances the activity of immune cells, and speeds up various metabolic processes that help fight infection.

The fever response begins when immune cells detect pathogens and release substances called pyrogens. These pyrogens travel through the bloodstream to the hypothalamus, where they trigger a reset of the body's temperature "thermostat." The hypothalamus then initiates various mechanisms to raise body temperature, including shivering (which generates heat through muscle activity), constriction of blood vessels near the skin (to reduce heat loss), and behavioral changes that make you want to bundle up and rest.

Understanding the underlying cause of fever is important because it determines the appropriate treatment approach. While most fevers are caused by self-limiting viral infections that resolve without specific treatment, some causes require medical intervention to prevent complications.

Viral Infections

Viral infections are the most common cause of fever worldwide. The common cold, caused by rhinoviruses and other respiratory viruses, frequently produces low-grade fever along with nasal congestion, sore throat, and cough. Influenza (the flu) typically causes higher fever, often above 39°C (102°F), accompanied by body aches, fatigue, and respiratory symptoms. COVID-19 has become another significant cause of fever, with temperature elevation being one of the most commonly reported symptoms of infection.

Other viral causes of fever include gastroenteritis (stomach flu), mononucleosis (caused by Epstein-Barr virus), and childhood viral illnesses such as roseola, chickenpox, and measles. Viral fevers typically last 2-4 days and resolve as the immune system successfully fights off the infection.

Bacterial Infections

Bacterial infections can cause fever ranging from mild to severe, depending on the type and location of infection. Urinary tract infections are a common cause of fever, particularly in women and older adults, and may be accompanied by painful urination, frequent urination, and lower abdominal discomfort. Pneumonia, whether bacterial or viral, often produces high fever along with cough, chest pain, and difficulty breathing.

Strep throat, caused by Group A Streptococcus bacteria, is characterized by fever, severe sore throat, and swollen lymph nodes. Unlike viral sore throats, strep throat requires antibiotic treatment to prevent complications such as rheumatic fever. Other bacterial causes include skin infections (cellulitis), sinus infections, ear infections, and more serious conditions like meningitis and sepsis.

Other Causes of Fever

While infections are the most common cause, fever can also result from non-infectious conditions. Inflammatory diseases such as rheumatoid arthritis and inflammatory bowel disease can cause fever as part of their inflammatory process. Certain autoimmune conditions, including lupus and vasculitis, may present with recurrent or persistent fever. Some cancers, particularly lymphomas and leukemias, can cause fever as a presenting symptom or ongoing manifestation of the disease.

Medications can sometimes cause drug-induced fever, either as a direct side effect or as part of an allergic reaction. Heat exhaustion and heat stroke can elevate body temperature dangerously. Recent vaccinations commonly cause low-grade fever as the immune system responds to the vaccine antigens – this is a normal and expected response that indicates the vaccine is stimulating immune protection.

What Are the Symptoms of Fever?

Fever symptoms include elevated body temperature (38°C/100.4°F or higher), chills and shivering when temperature is rising, sweating when temperature is falling, weakness, fatigue, headache, muscle aches, loss of appetite, and dehydration. The severity of symptoms often correlates with the degree of temperature elevation.

Recognizing fever involves more than just checking the thermometer. The experience of fever encompasses a constellation of symptoms that reflect the body's physiological response to elevated temperature. Understanding these symptoms helps you assess the severity of your condition and determine when medical care may be needed.

The most fundamental symptom of fever is, of course, elevated body temperature. However, the subjective experience of "feeling feverish" – that sense of being unwell combined with warmth – is often a reliable indicator even before temperature is measured. Many people can accurately sense when their temperature is elevated based on how they feel.

The physical sensations associated with fever follow a predictable pattern related to the body's temperature regulation. During the rising phase of fever, when the hypothalamus is actively increasing body temperature, you typically experience chills, shivering, and a strong desire to bundle up. This occurs because your body perceives its current temperature as "too cold" relative to the new, higher set point. Shivering generates heat through rapid muscle contractions, helping to raise your temperature to the new target.

Common Fever Symptoms

Beyond temperature elevation, fever commonly causes a range of symptoms that reflect the body's stress response and the metabolic demands of fighting infection. Fatigue and weakness are nearly universal, as the body diverts energy toward immune function. Headache is common and may be related to dehydration, changes in blood flow, or the direct effects of pyrogens on the brain.

Muscle aches and joint pain often accompany fever, particularly with viral infections like influenza. These symptoms result partly from the release of inflammatory cytokines – the same substances that trigger the fever response. Loss of appetite is typical, though maintaining adequate fluid intake remains crucial. Many people experience changes in taste and a general sense of malaise that makes eating unappealing.

• Elevated temperature: Core body temperature of 38°C (100.4°F) or higher
• Chills and shivering: Occur as body temperature is rising
• Sweating: Happens as fever "breaks" and temperature decreases
• Weakness and fatigue: General feeling of tiredness and low energy
• Headache: Often dull and persistent, worsened by dehydration
• Muscle aches: Generalized body pain, particularly common with flu
• Loss of appetite: Reduced desire to eat, though thirst may increase
• Dehydration: From sweating and reduced fluid intake
• Irritability: Especially noticeable in children with fever

When Temperature Falls

As the infection is brought under control and the hypothalamus resets to normal temperature, the body must now shed excess heat. This phase is characterized by sweating, sometimes profuse, as the body works to lower its temperature back to normal. The skin may become flushed and warm to the touch as blood vessels dilate to release heat. This "breaking" of fever is generally a positive sign indicating that the body is recovering from the infection.

What Is Normal Body Temperature?

Normal body temperature typically ranges from 36°C to 37.8°C (96.8°F to 100°F), with the classic "normal" of 37°C (98.6°F) representing an average. Temperature varies naturally throughout the day, being lowest in early morning and highest in late afternoon, and also varies by measurement method, age, and individual factors.

The concept of "normal" body temperature is more nuanced than many people realize. The oft-cited figure of 37°C (98.6°F) comes from 19th-century research by German physician Carl Reinhold August Wunderlich, who measured temperatures in thousands of patients. However, more recent research has suggested that average body temperature may actually be slightly lower than this historical standard, possibly around 36.6°C (97.9°F).

Temperature fluctuates naturally throughout the 24-hour cycle, a phenomenon known as circadian rhythm. Body temperature is typically lowest in the early morning hours, around 4-6 AM, and highest in the late afternoon or early evening, around 4-6 PM. This normal variation can span about 0.5°C (1°F) in healthy individuals. Understanding this pattern is important when interpreting temperature readings – a temperature of 37.5°C (99.5°F) in the early morning might represent a more significant elevation than the same reading in the evening.

Several factors influence what's "normal" for any individual. Women's body temperatures tend to fluctuate with the menstrual cycle, being higher after ovulation. Physical activity raises body temperature temporarily. Age plays a role as well – young children tend to have slightly higher baseline temperatures than adults, while older adults may have lower baseline temperatures and may not mount as robust a fever response to infection.

Measuring Body Temperature Accurately

The accuracy of temperature measurement depends significantly on the method used and proper technique. Different measurement sites yield different readings, and it's important to know what's "normal" for each method. Rectal temperatures are considered the gold standard, particularly for infants, as they most closely reflect core body temperature. Oral temperatures are convenient for adults and older children but can be affected by recent eating, drinking, or breathing through the mouth.

Ear (tympanic) thermometers measure infrared heat from the eardrum and can provide quick readings, though accuracy depends on proper positioning in the ear canal. Forehead (temporal artery) thermometers are non-invasive and easy to use, making them popular for children, though they may be less accurate in some situations. Axillary (armpit) temperatures are the least invasive but also least reliable, typically reading 0.5-1°C (1-2°F) lower than core temperature.

For the most accurate reading, rest for at least 30 minutes before taking your temperature. Avoid measuring immediately after physical activity, hot or cold drinks, or bathing. Take multiple readings if uncertain and note the time of day. Keep a record of your baseline temperature when healthy so you have a reference point for comparison when you're unwell.

When Should You See a Doctor for Fever?

Seek medical care for fever lasting more than 4 days, temperatures above 39.4°C (103°F), fever that doesn't respond to medication, or fever accompanied by severe symptoms like difficulty breathing, chest pain, confusion, stiff neck, or persistent vomiting. Infants under 3 months with any fever require immediate medical attention.

While most fevers are caused by self-limiting viral infections and resolve without specific treatment, certain situations require prompt medical evaluation. Knowing when to seek care can prevent complications and ensure appropriate treatment when needed. The decision to seek medical care depends on factors including the height and duration of fever, accompanying symptoms, and the patient's age and underlying health conditions.

Duration of fever is an important consideration. Most viral illnesses cause fever that peaks within the first 2-3 days and resolves within a week. Fever persisting beyond 4 days without improvement warrants medical evaluation, as it may indicate a bacterial infection requiring antibiotics, an unusual viral infection, or a non-infectious cause that needs investigation.

The height of fever matters as well. While fever itself is generally not dangerous at moderate levels, very high temperatures may indicate severe infection and can cause significant discomfort. Temperatures above 39.4°C (103°F) that don't respond to fever-reducing medications should prompt medical consultation. Temperatures above 40.5°C (105°F) require urgent evaluation.

Warning Signs Requiring Immediate Medical Attention

Certain symptoms accompanying fever indicate potentially serious conditions that need prompt evaluation. Difficulty breathing or shortness of breath may suggest pneumonia or other serious respiratory infection. Chest pain with fever could indicate heart involvement or severe lung infection. Confusion, unusual drowsiness, or difficulty waking someone with fever can be signs of serious infection affecting the brain or bloodstream.

A stiff neck accompanied by fever and headache is a classic warning sign of meningitis and requires emergency evaluation. Severe abdominal pain with fever may indicate appendicitis, gallbladder infection, or other surgical emergencies. A widespread rash developing with fever, particularly if it doesn't blanch (turn white) when pressed, can indicate serious bacterial infection like meningococcal disease.

Special Considerations by Age and Health Status

Infants under 3 months old with any fever (38°C/100.4°F or higher) require immediate medical evaluation, as serious bacterial infections can occur without other obvious symptoms in this age group. For infants 3-6 months, fever above 38.9°C (102°F) warrants medical consultation. Children between 6 months and 2 years with high fever lasting more than a day should be seen by a healthcare provider.

Adults with certain health conditions face higher risks from infections and should have a lower threshold for seeking care. Those with weakened immune systems (from HIV, cancer treatment, organ transplant medications, or other causes), chronic lung or heart disease, diabetes, or kidney disease should contact their healthcare provider early when fever develops. Pregnant women with fever should also seek medical advice promptly, as some infections can pose risks to the developing baby.

Fever occurring after recent surgery or invasive medical procedures requires evaluation, as it may indicate infection at the surgical site or elsewhere. Fever developing after travel to tropical or developing regions needs assessment for travel-related infections like malaria, dengue, or typhoid fever. Fever during or shortly after antibiotic treatment should be evaluated, as it may indicate treatment failure or a new infection.

How Can You Treat Fever at Home?

Home treatment for fever includes rest, staying well-hydrated with water and clear fluids, dressing lightly and keeping the room cool, and using over-the-counter medications like paracetamol (acetaminophen) or ibuprofen as directed. Avoid bundling up despite chills, and never use cold baths or alcohol rubs to reduce fever.

For most people with fever from common viral infections, home treatment focused on comfort and supportive care is appropriate and effective. The goal isn't necessarily to eliminate fever entirely – since fever serves a protective function – but rather to reduce discomfort and prevent complications like dehydration. Understanding proper home care techniques helps you manage fever safely and recognize when additional medical care might be needed.

Rest is fundamental to recovery. Your body needs energy to fight infection, and rest allows it to direct resources toward immune function. While you don't need to stay in bed continuously, avoid strenuous activities and give yourself permission to sleep when tired. Listen to your body's signals and don't push yourself to maintain normal activities when you're feverish and unwell.

Hydration deserves special emphasis. Fever increases fluid loss through sweating and faster breathing, while illness often reduces appetite for both food and drink. Dehydration can develop quickly, especially with high fever, making symptoms worse and prolonging recovery. Adults should aim to drink at least 2-3 liters of fluids daily during fever, more if sweating heavily. Water is ideal, but clear broths, diluted fruit juices, and electrolyte drinks are also appropriate. Avoid alcohol and caffeine, which can worsen dehydration.

Environmental and Physical Comfort Measures

Managing your environment helps maintain comfort during fever. Keep the room cool but not cold – around 20-22°C (68-72°F) is generally comfortable. Dress in light, breathable clothing rather than bundling up, even if you feel chilly. When chills occur during the rising phase of fever, it's natural to want to wrap up in blankets, but adding too many layers can trap heat and potentially raise body temperature further. Use light bedding that can be easily adjusted.

Cool compresses on the forehead, back of neck, or armpits can provide comfort, though they don't significantly reduce core body temperature. Avoid cold baths or ice packs, which can cause shivering (raising temperature) and may be uncomfortably shocking. Similarly, alcohol rubs are not recommended – alcohol evaporates quickly but provides no benefit over water and can be absorbed through the skin, particularly concerning in children.

A lukewarm (not cold) bath can help with comfort and may provide temporary relief. The water should feel comfortable, not shocking. Sponge baths with lukewarm water can be soothing, especially for children. Afterward, dress lightly and allow air circulation rather than bundling up immediately.

Over-the-Counter Medications for Fever

When fever causes significant discomfort, over-the-counter antipyretic (fever-reducing) medications can provide relief. The two main options are paracetamol (acetaminophen) and non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen. Both are effective at reducing fever and relieving associated symptoms like headache and muscle aches.

Paracetamol (acetaminophen) is generally the first-line choice for fever. It's effective, well-tolerated, and safe for most people when used as directed. Adults can typically take 500-1000mg every 4-6 hours, not exceeding 4000mg in 24 hours. Paracetamol is processed by the liver, so those with liver disease should use lower doses or avoid it. Never combine paracetamol with alcohol.

Ibuprofen and other NSAIDs reduce fever through different mechanisms and also provide anti-inflammatory effects. They're effective but should be taken with food to reduce stomach irritation. NSAIDs are not recommended for those with kidney disease, stomach ulcers, certain heart conditions, or during pregnancy (especially in the third trimester). They can also worsen asthma in some people.

Is Fever Different in Children?

Children often develop higher fevers than adults, and fever is especially common in young children as their immune systems encounter infections for the first time. While fever itself is rarely harmful, the underlying cause matters. Monitor for warning signs, ensure adequate hydration, and use age-appropriate fever medications. Never give aspirin to children.

Fever in children can be particularly concerning for parents, partly because children tend to develop higher temperatures than adults in response to the same infections. A child with a temperature of 39.5°C (103°F) from a common cold may actually be less ill than an adult with 38.5°C (101°F) from pneumonia. Understanding how fever differs in children helps parents respond appropriately without unnecessary panic or, conversely, dismissing warning signs.

The height of fever doesn't reliably indicate the severity of illness in children. Some serious bacterial infections may cause only modest fever, while benign viral infections often produce high temperatures. Instead of focusing solely on the thermometer reading, observe how your child is acting. A child with high fever who is still playful, drinking fluids, and responsive is generally less concerning than one with lower fever who is lethargic, refuses fluids, or seems unusually ill.

Young children often have fever more frequently than adults because their immune systems are still learning to recognize and fight common pathogens. Daycare and school attendance exposes children to numerous viruses, resulting in an average of 6-8 respiratory infections per year in young children. While this can feel overwhelming for parents, each infection helps build immunity that reduces infection frequency as children grow older.

Managing Fever in Children

The basic principles of fever management are similar for children and adults: ensure comfort, maintain hydration, and treat underlying causes when indicated. Children should rest but don't need strict bed rest. Encourage fluid intake – younger children may prefer popsicles, diluted juice, or clear broths in addition to water. Signs of adequate hydration include regular urination (at least every 6-8 hours), moist mouth, and tears when crying.

Dress children lightly and keep the room comfortable rather than bundling them up. Light, breathable clothing allows heat to dissipate. A child with fever who is shivering may need a light blanket temporarily, but remove extra layers once the shivering stops.

Fever medication isn't always necessary – if a child with fever is comfortable, drinking well, and sleeping normally, medication may not be needed. However, if fever causes significant discomfort, difficulty sleeping, or reduced fluid intake, fever-reducing medication can help. Use paracetamol (acetaminophen) or ibuprofen according to weight-based dosing instructions. Ibuprofen is not recommended for infants under 6 months or children who are dehydrated.

How Does the Immune System Create Fever?

The immune system creates fever through a coordinated response involving white blood cells, signaling molecules called pyrogens, and the hypothalamus (the brain's thermostat). When infection is detected, immune cells release cytokines that signal the hypothalamus to raise the body's temperature set point, triggering shivering and heat conservation to achieve the higher temperature.

Understanding the mechanism behind fever helps explain why it's generally a beneficial response and why treatment focuses on comfort rather than aggressively suppressing temperature. Fever is not a malfunction but rather a precisely regulated response that has evolved over millions of years as a defense against infection.

The fever response begins when the immune system detects the presence of pathogens. White blood cells, particularly macrophages and monocytes, recognize foreign invaders through pattern recognition receptors that identify molecular patterns common to many pathogens. Upon activation, these immune cells release a variety of signaling molecules called cytokines, including interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor (TNF).

These cytokines travel through the bloodstream to the hypothalamus, the brain region that serves as the body's thermostat. Within the hypothalamus, cytokines trigger the production of prostaglandins, particularly prostaglandin E2 (PGE2). These prostaglandins act directly on temperature-regulating neurons, effectively raising the thermostat's set point. Interestingly, this is why NSAIDs like ibuprofen are effective fever reducers – they block prostaglandin production.

Once the hypothalamus establishes a higher set point, the body perceives its current temperature as "too cold" and activates heat-generating and heat-conserving mechanisms. Blood vessels in the skin constrict to reduce heat loss, producing the pale, cold skin often seen at the onset of fever. Shivering generates heat through rapid muscle contractions. Behavioral changes – wanting to curl up under blankets, feeling cold – reinforce these physiological responses.

Why Fever Helps Fight Infection

The elevated temperature during fever provides several advantages against pathogens. Many bacteria and viruses have optimal growth temperatures around normal body temperature; even modest increases can significantly impair their ability to replicate. Some studies suggest that fever can reduce viral replication rates by 200-fold for certain viruses.

Higher body temperature also enhances immune function. White blood cells migrate more efficiently to sites of infection at elevated temperatures. The production of antibodies and other immune proteins is increased. Certain immune cells become more effective at destroying pathogens. The overall metabolic rate increases, speeding up the many biochemical reactions involved in mounting an immune response.

Research in both animal models and human studies suggests that allowing moderate fever to run its course may actually shorten the duration of some infections. However, this doesn't mean fever should never be treated – comfort and functionality matter, and very high fevers can be dangerous. The key is balanced management that reduces suffering while not completely suppressing the body's natural defense response.