Pyrexia Explained: Your Guide to Understanding Common Fever
Know about the pyrexia, what it is, the process of fever, causes, symptoms, types, and when to seek a doctor. Learn about the management and treatment options, pyrexia in specific populations and more.
Written by Dr. Vasanthasree Nair
Reviewed by Dr. Dhankecha Mayank Dineshbhai MBBS
Last updated on 5th Sep, 2025
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Introduction
That familiar feeling of warmth, the chills, the general sense of malaise, a fever, or pyrexia, is one of the most common health experiences, yet it often causes significant worry. This guide demystifies pyrexia, breaking down everything you need to know about your body's built-in alarm system. We'll explore what a fever truly is, why it happens, the various types you might encounter, and most importantly, how to manage it safely at home and recognise when it's time to seek professional medical advice.
What is Pyrexia? Defining a Fever
Pyrexia is the medical term for what we commonly call a fever. It is not an illness in itself but rather a symptom, a physiological response triggered by your body. Technically, it's defined as a temporary increase in your body's core temperature, surpassing the normal daily fluctuation.
The widely accepted average normal body temperature is 98.6°F (37°C). However, this is just a benchmark. A normal temperature can naturally range from about 97°F (36.1°C) to 99°F (37.2°C).
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How Your Body Regulates Temperature: The Hypothalamus
To understand fever, you must first understand thermoregulation. Your body meticulously maintains its core temperature within a narrow, healthy range through a process called thermoregulation. The master conductor of this process is the hypothalamus.
- Normal State: The hypothalamus keeps your body's "set point" at around 98.6°F. It receives signals from temperature receptors throughout the body and initiates responses like sweating to cool down or shivering to warm up.
- Febrile State: When pyrogens (fever-producing substances) enter the bloodstream, they signal the hypothalamus to raise the body's set point. Your brain now believes a higher temperature is necessary to fight a threat. In response, you might feel chills and shiver to generate heat, and your blood vessels constrict to conserve heat, making you feel cold despite your rising temperature.
Common Causes of Fever: Why It Happens
A fever is a hallmark sign of an underlying issue. Your body raises its temperature to create a less hospitable environment for pathogens and to kick your immune system into high gear.
The most common causes of fever include:
- Infections: This is the overwhelming majority of cases. This includes:
- Viral infections: Influenza (the flu), common cold, COVID-19, RSV.
- Bacterial infections: Strep throat, urinary tract infections (UTIs), pneumonia, ear infections.
- Inflammatory Conditions: Diseases like rheumatoid arthritis or inflammatory bowel disease can cause fever.
- Medications: Some drugs, particularly antibiotics and those used to treat high blood pressure or seizures, can cause a drug-induced fever.
- Immunisations: A low-grade fever is a common and normal reaction in children and some adults after vaccinations.
- Heat Exhaustion or Heat Stroke: Caused by prolonged exposure to high temperatures.
- Other Causes: Certain cancers, autoimmune diseases, and blood clots.
Types of Fever: Patterns and What They Mean
Not all fevers are the same. Doctors sometimes classify them based on their pattern and duration, which can offer clues to the underlying cause.
- Intermittent Fever: The temperature spikes but returns to normal at least once within 24 hours. This is common with malaria.
- Remittent Fever: The temperature fluctuates but never returns to the normal range. This is very common with viral illnesses.
- Sustained or Continuous Fever: The temperature remains consistently above normal with little fluctuation (less than 0.3°C or 0.5°F per day). This can be seen with pneumonia.
- Hectic Fever: Characterised by large swings in temperature, often with sweating and chills, occurring within a day.
- Relapsing Fever: This pattern involves episodes of fever that last for a few days, followed by periods of normal temperature for a few days, before the fever returns.
Symptoms That Accompany a Fever
A fever rarely comes alone. It's usually part of a constellation of symptoms that together paint a clearer picture of the illness. Common symptoms of fever include:
- Chills, shivering, and feeling cold
- Sweating and flushed skin
- Headache and body aches (myalgia)
- General weakness and fatigue (malaise)
- Loss of appetite
- Dehydration
- Irritability, especially in children
In infants, be alert for symptoms like lethargy, inconsolable crying, poor feeding, and difficulty breathing.
How to Accurately Take a Temperature
Getting an accurate reading is crucial for proper management. The method you use depends on age and convenience.
- Rectal (for infants): Considered the gold standard for accuracy in babies.
- Oral (for children and adults): Accurate and common. Wait 15 minutes after eating or drinking.
- Tympanic (ear): Fast and convenient, but can be less accurate if not positioned correctly.
- Temporal artery (forehead): A good non-invasive option, though readings can be affected by sweat.
- Axillary (armpit): The least accurate method, generally not recommended.
Digital thermometers are the standard today, having replaced older mercury glass thermometers for safety reasons.
When to Seek Medical Attention for a Fever
This is the most critical question for most people. While many fevers can be managed at home, certain red flags require immediate medical advice.
For Infants and Young Children
- Under 3 months: Any temperature of 100.4°F (38°C) or higher requires immediate medical attention.
- 3-6 months: A temperature above 102°F (38.9°C) or a lower fever accompanied by unusual irritability, lethargy, or discomfort.
- Any age: A fever that lasts more than 24-48 hours, is accompanied by a rash, signs of dehydration (no wet diapers, sunken eyes), seizure, or a stiff neck.
For Adults
Seek care if a fever is:
- 103°F (39.4°C) or higher.
- Accompanied by severe headache, rash, sensitivity to light, confusion, persistent vomiting, chest pain, or difficulty breathing.
- Lasting more than three days.
- Occurring with a known serious illness (e.g., cancer, HIV) or a compromised immune system.
Home Remedies and Management of Fever
For a mild fever in an otherwise healthy individual, home care is often sufficient. The goal is comfort, not necessarily to eliminate the fever entirely, as it is serving a purpose.
- Stay Hydrated: Fever increases fluid loss. Drink water, broth, or electrolyte solutions. Preventing dehydration is paramount.
- Rest: Your body needs energy to fight the infection. Prioritise sleep and reduce activity.
- Stay Comfortably Cool: Dress in lightweight clothing and use a light blanket if you have chills. A lukewarm sponge bath can provide comfort (avoid cold water or ice baths, which can cause shivering and raise temperature further).
- Over-the-Counter (OTC) Medications: Fever reducers like acetaminophen (Tylenol) or ibuprofen (Advil, Motrin) can reduce fever and alleviate aches. Always follow dosage instructions carefully and be aware of any other medications you are taking to avoid overdose, particularly with acetaminophen.
Medications for Fever: What to Use
Understanding your options for fever treatment is key to safe management.
- Acetaminophen (Tylenol): Effective for reducing fever and pain. It is generally gentler on the stomach but must be used with caution as an overdose can cause severe liver damage. Do not exceed the recommended dose.
- Ibuprofen (Advil, Motrin) and Naproxen (Aleve): These are NSAIDs (Nonsteroidal Anti-inflammatory Drugs). They reduce fever, pain, and inflammation. They can cause stomach upset and are not recommended for people with certain kidney conditions or ulcers.
- Aspirin: While effective, aspirin should never be given to children or teenagers recovering from chickenpox or flu-like symptoms due to the risk of Reye's syndrome, a rare but serious condition.
Important: Always consult a doctor or pharmacist before giving medication to a child, especially for the first time.
Fever in Specific Populations
Fever in Children
Fevers are very common in children and are often their immune system's robust response to an infection. The number on the thermometer is less important than the child's behaviour. A child with a 104°F fever who is drinking fluids and alert is often less concerning than a child with a 101°F fever who is lethargic and inconsolable.
Fever in the Elderly
Older adults may not develop a high fever even with a serious infection. A slight elevation in temperature, or even a lower-than-normal temperature, can be a sign of illness. Any change in baseline condition warrants attention.
Complications of Untreated High Fever
While rare, a very high fever (generally above 107.6°F or 42°C) that is left untreated can lead to complications like febrile seizures in young children (usually benign but frightening to witness) or, in extreme cases, confusion, hallucinations, and organ damage. This underscores the importance of monitoring a fever and knowing when to seek help.
Prevention: Reducing the Risk of Infection
- The best way to avoid a fever is to prevent the illness that causes it.
- Practice good hand hygiene: Wash hands frequently with soap and water.
- Get vaccinated: Stay up-to-date on recommended vaccines, including the annual flu shot.
- Avoid close contact with sick individuals.
- Maintain a healthy lifestyle with a balanced diet, regular exercise, and adequate sleep to support a strong immune system.
Conclusion
Understanding pyrexia empowers you to respond appropriately instead of reacting with fear. A common fever is a sign that your body's sophisticated defense system is actively at work, fighting off an invader. By learning to accurately measure a temperature, provide supportive care at home, and, crucially, recognise the warning signs that necessitate a doctor's visit, you can navigate this universal health experience with confidence.
Consult a General Practitioner for Personalised Advice
Consult a General Practitioner for Personalised Advice
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Consult a General Practitioner for Personalised Advice
Dr. Gaddam Manoj
General Practitioner
1 Years • MBBS
Hyderabad
Aaradhya clinic, Hyderabad
Dr Syed Mateen Pasha
General Physician
2 Years • MBBS
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. Anand Ravi
General Physician
2 Years • MBBS
Bengaluru
PRESTIGE SHANTHINIKETAN - SOCIETY CLINIC, Bengaluru
Dr. D Bhanu Prakash
General Practitioner
10 Years • MBBS, AFIH, Advanced certificate in critical care medicine, Fellowship in critical care medicine
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Apollo 24|7 Clinic, Hyderabad
Dr. Madhuri Sai Sreepada
General Practitioner
9 Years • MBBS
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