Pyrexia: Common Fever Explained
Comprehensive guide to pyrexia (fever): causes, symptoms, measurement, home care, red flags, treatment, and when to consult a doctor. Learn safe ways to manage fever in adults and children.
Written by Dr. J T Hema Pratima
Reviewed by Dr. D Bhanu Prakash MBBS, AFIH, Advanced certificate in critical care medicine, Fellowship in critical care medicine
Last updated on 25th Oct, 2025
Introduction
A fever—also called pyrexia—is one of the most common reasons people seek health advice. It can feel alarming to watch the thermometer climb or to see a child flushed and fatigued. Yet fever is a natural response that often helps your body fight infection. In this guide, we’ll explain what counts as a fever, why it happens, how to measure it accurately, and what to do at home. We’ll cover common causes in adults and children, when to seek medical care, what tests doctors may order, and special situations like travel-related illnesses or fevers during pregnancy. You’ll also learn how to prevent common fevers and how to tell myths from facts. Throughout, we’ll use friendly, plain language and point you to trusted sources, with practical tips you can use today. If symptoms persist or you’re unsure, you’ll find guidance on when to consult a clinician, including convenient options through Apollo 24|7. By the end, you’ll be confident navigating common fever questions for yourself and your family.
What is a Fever (Pyrexia)?
Here’s what you need to know about fever and how it develops:
Normal body temperature and what “fever” means
Most adults have a normal oral temperature around 37°C (98.6°F), but healthy ranges vary throughout the day and from person to person, typically 36.1–37.2°C (97–99°F). Fever is generally defined as 38.0°C (100.4°F) or higher measured orally, or 38.0°C (100.4°F) rectally in infants and children. Low-grade fever often refers to 37.5–38.0°C (99.5–100.4°F), while high fever can be 39.0–40.0°C (102.2–104°F). A temperature above 41.0°C (105.8°F) is rare and can be dangerous.
How the hypothalamus resets your set point
Fever is a controlled rise in body temperature triggered by your immune system. When microbes or inflammation release pyrogens, your hypothalamus raises the body’s set point, prompting shivering and reduced skin blood flow to reach the new target. This differs from overheating in hot environments; fever is internally regulated and purposeful.
Fever vs hyperthermia vs heat exhaustion
Hyperthermia (e.g., heat stroke) is overheating when the body cannot shed heat—an emergency that can be fatal—without a hypothalamic set-point change. Clues include hot, dry skin, confusion, and exposure to heat. Fever, by contrast, usually involves chills and sweats as the set point rises and falls.
Is Fever Good or Bad? What Your Body Is Trying to Do
Here’s how fever can affect your body and when it may be risky:
Benefits of fever for fighting infections
Moderate fever may help immune cells work better and make the environment less hospitable to some pathogens. Many viral fevers resolve on their own within 3–5 days as your immune system clears the infection.
When fever can be harmful
Prolonged, very high fevers can cause dehydration, weakness, and rarely complications such as confusion. In vulnerable groups—newborns, older adults, pregnant people, and those with chronic illnesses—fever warrants earlier evaluation. Fever itself is uncomfortable but seldom dangerous; the cause of fever is what needs attention. If symptoms persist beyond about 3 days in adults or your child seems very unwell at any time, seek medical advice. If symptoms persist beyond two weeks, consult a doctor online with Apollo 24|7 for further evaluation.
Consult Top Specialists
Common Causes of Fever
Here’s an overview of the most frequent fever triggers:
Viral and bacterial infections
Viral infections are the most common cause of fever: colds, influenza, COVID-19, and gastroenteritis often present with low-to-moderate fever, sore throat, cough, or diarrhoea. Bacterial infections like strep throat, urinary tract infections, pneumonia, or skin infections typically cause localised symptoms plus fever; they may require antibiotics.
Non-infectious causes (autoimmune, cancers, medications)
Autoimmune conditions (lupus, rheumatoid arthritis), inflammatory disorders, certain cancers (like lymphomas), blood clots, thyroid storm, and drug reactions can trigger fever. Some medicines (e.g., certain antibiotics, anticonvulsants) cause drug fever that stops when the medication is discontinued under guidance.
Post-vaccination fever and what to expect
Mild fever within 24–48 hours after vaccination is common and usually self-limited; it signals an active immune response. Manage with fluids and rest; use antipyretics if needed per age-appropriate dosing.
Fever in Children vs Adults: What’s Different
Here’s what parents and adults should know about age-based fever concerns:
Age-based thresholds and red flags
Infants under 3 months with a rectal temperature of 38.0°C (100.4°F) or higher need urgent medical evaluation due to higher risk of serious infection. In children 3–36 months, a fever lasting more than 24–48 hours or any fever with poor feeding, unusual sleepiness, persistent vomiting, or rash should prompt medical advice. For adults, seek care if fever exceeds 39.4°C (103°F), lasts more than 3 days, or is accompanied by chest pain, shortness of breath, severe headache, stiff neck, confusion, or persistent vomiting.
Febrile seizures: risk, safety, and next steps
About 2–5% of children between 6 months and 5 years may have a febrile seizure, often with a sudden temperature rise. These are usually brief and not linked to epilepsy or brain damage. Place the child on their side, don’t restrain, don’t put anything in the mouth, and seek prompt medical evaluation the first time to rule out serious causes.
Symptoms That Travel With Fever (and Red Flags)
Here’s what symptoms often accompany fever and when to act:
Typical associated symptoms
Chills, sweats, body aches, headache, sore throat, cough, runny nose, nausea, diarrhoea, and fatigue commonly accompany fever and may point to the cause. Dehydration is a risk: watch for dry mouth, dark urine, dizziness, or in children, fewer wet diapers.
When to go to urgent care or emergency services
Seek urgent help for fever with chest pain, shortness of breath, confusion, stiff neck, severe headache, persistent vomiting, purple rash, severe abdominal pain, one-sided weakness, or signs of heat stroke (hot dry skin, collapse), or if a newborn has any fever. Adults with immune suppression (e.g., chemotherapy, transplant) should seek immediate care for any fever.
How to Measure a Fever Correctly?
Here’s how to get an accurate temperature reading:
Thermometer types and best practices
Digital thermometers are preferred. Rectal temperatures are most accurate for infants and small children; axillary (underarm) is less accurate but useful for screening. Oral thermometers work for older children and adults if used correctly (no hot/cold drinks 15 minutes before). Tympanic (ear) and temporal (forehead) devices are convenient but technique-sensitive.
Common mistakes to avoid
Avoid mercury thermometers. Don’t compare different sites directly (rectal tends to read higher than oral/axillary). Measure at rest, not after exercise or a hot bath. Clean the device after each use per manufacturer’s instructions.
Home Care: Safe Ways to Bring Down a Common Fever
Here’s practical guidance for managing fever at home:
Rest, fluids, light clothing, and tepid sponging
Rest supports recovery; sip water, oral rehydration solutions, or broths to replace fluids. Wear light clothing and keep the room comfortably cool. Tepid sponging can provide short-term relief but offers limited added benefit beyond fever reducers and may cause shivering if water is too cool; avoid ice water or alcohol rubs.
Medications: acetaminophen and ibuprofen worked out safely
Acetaminophen (paracetamol) and ibuprofen reduce fever and discomfort when dosed by weight and timing. For children, use age-appropriate formulations and dosing syringes. Avoid aspirin in anyone under 19 due to Reye’s syndrome risk. If you have liver disease (acetaminophen) or kidney disease/stomach ulcers (ibuprofen), ask your clinician first. It’s okay to prioritise comfort; you don’t need to chase every low-grade fever.
What to avoid (aspirin in kids, alcohol rubs)
Do not use aspirin in children/teens; avoid alcohol rubs and cold baths. Focus on hydration, light layers, and rest.Consult Top Specialists
When to See a Doctor and What to Expect?
Here’s guidance on seeking medical advice:
Time-based rules and high-risk groups
Adults: see a clinician if fever persists >3 days, is ≥39.4°C (103°F), or comes with concerning symptoms (chest pain, confusion, severe headache, stiff neck). Children: infants under 3 months with any fever; 3–36 months if fever lasts >24–48 hours or child looks unwell; any age with red flags. High-risk groups (pregnant people, older adults, immunocompromised, recent surgery) should seek earlier evaluation.
Apollo 24|7 options for online consults and home tests
If your fever persists or you’re unsure about next steps, consult a doctor online with Apollo 24|7 for guidance and prescriptions. If lab tests are advised (CBC, malaria/dengue antigen, COVID-19, urine analysis, chest X-ray referral), Apollo 24|7 offers convenient home collection for tests like CBC, CRP, dengue NS1/IgM, malaria, and urine culture.
Diagnosis and Tests: How Clinicians Evaluate a Persistent Fever
Here’s what to expect during medical evaluation:
Basic labs and cultures
Depending on your symptoms and risk, common initial tests include complete blood count (CBC), C-reactive protein (CRP) or ESR, basic metabolic panel, urinalysis/urine culture, throat swab for strep, and chest X-ray if cough or chest symptoms are present. For travel-related fever, rapid malaria tests and dengue/typhoid serologies may be appropriate.
Imaging and special testing
When indicated, clinicians may order ultrasound or CT scans to look for hidden abscesses or infections, stool tests if gastrointestinal symptoms are prominent, or autoimmune panels if inflammatory disease is suspected.
Pyrexia of unknown origin (PUO) overview
PUO refers to fever >38.3°C (101°F) on several occasions lasting >3 weeks without a diagnosis after initial evaluation. Causes include atypical infections, autoimmune diseases, malignancies, and drug reactions. Workup is stepwise and tailored to clues from history and exam.
Treatment Beyond Home Care
Here’s what medical management may involve:
Antimicrobials and targeted therapy
If a bacterial infection is confirmed or strongly suspected, your clinician may prescribe antibiotics; for influenza in high-risk patients, early antivirals can shorten illness. Not all fevers require antibiotics—most viral causes resolve without them. Overuse can cause side effects and resistance.
Hospital care, fluids, and monitoring
Severe dehydration, sepsis, pneumonia with low oxygen, or confusion may require intravenous fluids, oxygen, and close monitoring. Clinicians will treat the underlying cause while managing discomfort with antipyretics.
Special Situations
Here’s how fever can differ in specific contexts:
Travel-related fever (malaria, dengue, typhoid)
Fever within weeks of travel to malaria-endemic or dengue regions requires urgent evaluation to rule out serious illnesses; malaria can rapidly worsen. Share your itinerary, mosquito exposure, food/water habits, and vaccination history. Use repellents and bed nets during travel to reduce risk.
Pregnancy, older adults, and immunocompromised patients
In pregnancy, fever can signal infection; prompt assessment helps protect both parent and baby. Older adults may have lower baseline temperatures and may not mount high fevers even in serious infection—look for confusion or weakness. Immunocompromised people should treat any fever as urgent.
Fever after vaccination
Mild, short-lived fever is common post-immunisation; manage as above. High or persistent fever, or concerning symptoms (e.g., breathing difficulty), warrant medical advice.
Preventing Common Fevers
Here’s practical advice to reduce the likelihood of fever:
Hygiene, vaccines, vector control
Wash hands regularly, avoid touching your face, cover coughs, and disinfect high-touch surfaces. Keep vaccinations up to date (influenza, COVID-19, pneumococcal, childhood vaccines) to prevent
Food and water safety while travelling
Drink safe water, eat thoroughly cooked foods, peel fruits/vegetables, and avoid raw seafood in high-risk areas. Consider travel vaccines and malaria prophylaxis when appropriate.
Myths and Misconceptions About Fever
Here’s clarification on common fever myths:
“High number = severe disease”
The thermometer number alone doesn’t determine severity; the overall condition and symptoms matter more. Some mild viral infections cause higher fevers than bacterial ear infections, for example.
“You must break every fever”
Fever can be helpful; treat primarily for comfort and hydration. There’s no evidence that treating a fever always shortens illness, and tepid sponging has limited benefit beyond medications. Avoid overmedicating, and follow dosing guidance carefully.
Outlook and Recovery: Getting Back to Normal
Here’s what to expect after a fever:
Safe return to work/school
Return when fever-free for 24 hours without fever-reducing medicine and when you feel well enough to participate. For some infections (e.g., influenza, COVID-19), follow local public health guidance regarding isolation.
Monitoring and follow-up
Track temperature trends and symptoms rather than checking constantly. If your condition does not improve after trying these methods, book a physical visit to a doctor with Apollo 24|7. Apollo 24|7 offers convenient home collection for tests like CBC or urinalysis if your clinician recommends follow-up.
Conclusion
Fever—often called pyrexia—is among the most common and misunderstood symptoms. Knowing what temperature counts as a fever, how to measure it correctly, and how to care for yourself or your child at home can reduce anxiety and speed recovery. Most fevers are caused by viral infections and settle within a few days. Focus on rest, fluids, and comfort, and watch for red flags such as breathing difficulty, confusion, severe headache, a stiff neck, or a rash that doesn’t blanch. Infants under three months with any fever and high-risk groups should be assessed promptly. When fever persists beyond a few days, or when you’re unsure of the cause, clinicians can guide testing to rule out infections like urinary tract infections, pneumonia, or travel-related illnesses such as malaria or dengue. If symptoms persist beyond two weeks, consult a doctor online with Apollo 24|7 for further evaluation, and consider home collection for recommended lab tests. With the right information and support, you can make sensible, safe decisions about common fevers, from home care to timely medical attention.Consult Top Specialists
Consult Top Specialists
Dr. Rajib Ghose
General Physician/ Internal Medicine Specialist
25 Years • MBBS
East Midnapore
VIVEKANANDA SEBA SADAN, East Midnapore
Dr. Sougata Kumar
General Practitioner
8 Years • MBBS
East Midnapore
VIVEKANANDA SEBA SADAN, East Midnapore
Dr. Santanu Mandal
General Physician/ Internal Medicine Specialist
17 Years • MD (Physician), DNB (General Medicine)
Kolkata
MCR SUPER SPECIALITY POLY CLINIC & PATHOLOGY, Kolkata
(25+ Patients)
Dr. Swagata Sircar
General Physician/ Internal Medicine Specialist
8 Years • MBBS, MD General Medicine
Kolkata
HealthYou Speciality Clinic & Diagnostics., Kolkata
Dr Aakash Andgi
General Physician/ Internal Medicine Specialist
9 Years • MBBS MD
Bengaluru
Apollo Clinic, JP nagar, Bengaluru
Consult Top Specialists
Dr. Rajib Ghose
General Physician/ Internal Medicine Specialist
25 Years • MBBS
East Midnapore
VIVEKANANDA SEBA SADAN, East Midnapore
Dr. Sougata Kumar
General Practitioner
8 Years • MBBS
East Midnapore
VIVEKANANDA SEBA SADAN, East Midnapore
Dr. Santanu Mandal
General Physician/ Internal Medicine Specialist
17 Years • MD (Physician), DNB (General Medicine)
Kolkata
MCR SUPER SPECIALITY POLY CLINIC & PATHOLOGY, Kolkata
(25+ Patients)
Dr. Swagata Sircar
General Physician/ Internal Medicine Specialist
8 Years • MBBS, MD General Medicine
Kolkata
HealthYou Speciality Clinic & Diagnostics., Kolkata
Dr Aakash Andgi
General Physician/ Internal Medicine Specialist
9 Years • MBBS MD
Bengaluru
Apollo Clinic, JP nagar, Bengaluru
Consult Top Specialists
Dr. Rajib Ghose
General Physician/ Internal Medicine Specialist
25 Years • MBBS
East Midnapore
VIVEKANANDA SEBA SADAN, East Midnapore
Dr. Sougata Kumar
General Practitioner
8 Years • MBBS
East Midnapore
VIVEKANANDA SEBA SADAN, East Midnapore
Dr. Santanu Mandal
General Physician/ Internal Medicine Specialist
17 Years • MD (Physician), DNB (General Medicine)
Kolkata
MCR SUPER SPECIALITY POLY CLINIC & PATHOLOGY, Kolkata
(25+ Patients)
Dr. Swagata Sircar
General Physician/ Internal Medicine Specialist
8 Years • MBBS, MD General Medicine
Kolkata
HealthYou Speciality Clinic & Diagnostics., Kolkata
Dr Aakash Andgi
General Physician/ Internal Medicine Specialist
9 Years • MBBS MD
Bengaluru
Apollo Clinic, JP nagar, Bengaluru
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Frequently Asked Questions
1. What temperature is considered a fever in adults?
In adults, 38.0°C (100.4°F) or higher is generally considered a fever. Seek care if it lasts more than 3 days or reaches 39.4°C (103°F), or if you develop red flags like chest pain or confusion.
2. How can I reduce fever naturally at home?
Rest, hydrate, wear light clothing, keep the room cool, and try tepid sponging. Use acetaminophen or ibuprofen for comfort if needed, following dosing guidance.
3. When should I be worried about a child’s fever?
Any fever in a baby under 3 months needs urgent evaluation. For older children, seek advice if fever lasts more than 24–48 hours, is very high, or the child seems unusually sleepy, irritable, has trouble breathing, a stiff neck, or a rash.
4. Can teething cause a fever?
Teething may cause a slight rise in temperature but not true fever (≥38.0°C/100.4°F). If your child has a real fever, look for other causes such as viral infections and seek guidance if concerned.
5. Do I need antibiotics for a fever?
Not usually. Most fevers are viral and improve on their own. Antibiotics help only with bacterial infections diagnosed by a clinician. If your condition does not improve after home care, book a visit with Apollo 24|7.