Fever is one of the most common signs that something is wrong in your body. It is not an illness in itself. It shows that your body is fighting an infection, inflammation, or another health problem. In this detailed guide, you will learn what a fever is, what causes it, how to measure it, how to treat it at home, and when you need a doctor.
What Is a Fever?
Your brain controls your body temperature in a small area called the hypothalamus. It works like a thermostat. When germs enter your body, your immune system releases chemicals called pyrogens, and these raise the set point of the thermostat. Your body then shivers and keeps heat until your temperature reaches the new level.
A normal body temperature is about 37°C (98.6°F), but it changes by about 0.5°C during the day. It is lower in the morning and higher in the late afternoon. A temperature of 38°C (100.4°F) or higher is usually a fever.
Temperature Ranges
- Normal: about 36.1°C to 37.2°C (97°F to 99°F)
- Low-grade fever: 37.5°C to 38.3°C (99.5°F to 101°F)
- Fever: 38°C (100.4°F) or higher
- High fever: 39.4°C (103°F) or higher
- Dangerous fever: 41.5°C (106.7°F) or higher, which needs emergency care
Fever and Heat Illness
A fever is different from heat stroke. In heat stroke, the body overheats because of hot surroundings, and the brain’s thermostat does not change. Heat stroke is an emergency.
Why Do We Get a Fever?
A fever is a defense. Many germs grow best at normal body temperature, and a higher temperature slows them down. Heat also helps your immune cells work faster. This is why a mild fever is not always harmful and does not always need treatment.
Causes of Fever
Infections
Infections are the most common cause.
- Viral infections: colds, flu, COVID-19, dengue, measles, and chickenpox
- Bacterial infections: typhoid, tuberculosis, pneumonia, urinary tract infections, throat infections, and ear infections
- Parasitic infections: malaria
Inflammatory Conditions
Diseases such as rheumatoid arthritis, lupus, and inflammatory bowel disease can cause fever.
Other Causes
- Vaccines: A mild fever for a day or two after a vaccine is common.
- Medicines: Some antibiotics, seizure medicines, and other drugs cause drug fever.
- Heat exposure: Too much sun or a hot environment can raise body temperature.
- Teething: It can cause a slight rise in temperature in babies, but not a high fever.
- Cancer: Some cancers can cause fever that lasts for weeks. Doctors check for them when a fever has no clear cause. For example, lymphoma can cause long fevers with night sweats and weight loss.
Symptoms That Come With Fever
- Sweating and chills
- Shivering
- Headache
- Muscle and joint aches
- Weakness and tiredness
- Loss of appetite
- Dehydration
- Irritability in children
- Flushed, warm skin
How to Measure a Fever
You can measure temperature in different places. Use a digital thermometer.
- Mouth: Wait 15 minutes after eating or drinking. Place the tip under the tongue and close your lips.
- Armpit: Place the tip in the armpit and press your arm against your body. This method is the least accurate. Add about 0.5°C to the reading.
- Rectum: This is the most accurate method for babies. Use a lubricated, separate thermometer.
- Ear and forehead: These are fast, but position changes the result.
Do not use old mercury thermometers.
Fever in Children, Adults, and Older People
Babies and Children
Fever is common in children and is usually caused by viral infections. Pay more attention to how the child behaves than to the number. A child who plays, drinks, and smiles is usually less ill than a child who is very sleepy or refuses to drink.
Adults
Adults usually have a fever from infections. A temperature above 39.4°C (103°F) or one lasting more than three days needs a doctor.
Older Adults
Older adults may have a serious infection with only a mild fever or no fever. Confusion or weakness can be the only signs.
How to Treat a Fever at Home
You do not always need to lower a fever. Treat it when it makes you feel unwell.
- Rest: Your body needs energy to fight infection.
- Drink fluids: Fever makes you lose water through sweat. Drink water, oral rehydration solution, soup, and juice. Babies should keep breastfeeding or taking formula.
- Eat light food: Eat small amounts if you have an appetite. Do not force food.
- Dress lightly: Use a light blanket. Too many clothes hold heat. If you feel cold, add a layer, and remove it when the chills stop.
- Keep the room comfortable: Keep the room cool and well aired.
- Use a lukewarm sponge: Sponge the body with lukewarm water. Do not use cold water, ice, or rubbing alcohol. They make you shiver and can raise your temperature.
Medicines for Fever
- Paracetamol (acetaminophen): It lowers fever and eases aches. Adults usually take 500 mg to 1,000 mg every four to six hours, up to 4,000 mg in 24 hours. Doctors dose children by weight, usually 10 to 15 mg per kilogram every four to six hours.
- Ibuprofen: It lowers fever and reduces inflammation. Take it with food. Do not give it to babies under 6 months unless a doctor advises it.
- Aspirin: Do not give aspirin to children or teenagers under 16. It can cause Reye’s syndrome, a serious illness affecting the liver and brain.
Do not give two medicines with the same ingredient. Check cold and flu products for hidden paracetamol.
Febrile Seizures
Some children aged 6 months to 5 years have a seizure when their temperature rises quickly. A febrile seizure looks frightening, but it is usually harmless and short. During a seizure:
- Lay the child on one side on a soft surface.
- Do not put anything in the mouth.
- Time the seizure.
- Call emergency services if it lasts more than 5 minutes, if it is the first seizure, or if the child does not wake up.
Fever medicine does not prevent febrile seizures.
When to See a Doctor
- Babies under 3 months: Any temperature of 38°C (100.4°F) or higher is an emergency.
- Babies 3 to 6 months: See a doctor for a temperature above 38.9°C (102°F) or any fever with unusual sleepiness.
- Children: See a doctor if the fever lasts more than two to three days, if the child refuses to drink, has a rash, or is very sleepy or irritable.
- Adults: See a doctor if your temperature is 39.4°C (103°F) or higher, or if the fever lasts more than three days.
- Any age: Get emergency help for a stiff neck, severe headache, confusion, trouble breathing, chest pain, seizures, repeated vomiting, a rash that does not fade when pressed, or a fever after travel to a malaria or dengue area.
How Doctors Find the Cause
Doctors ask about your symptoms, travel, and contacts with sick people. They examine you and may order:
- Blood tests, such as a complete blood count, malaria test, dengue test, or typhoid test
- Urine tests
- Chest X-ray
- Throat swab or other cultures
When a fever lasts more than three weeks with no clear cause, doctors call it a fever of unknown origin and look for hidden infections, inflammatory diseases, or cancers.
How to Prevent Fever
- Wash your hands often with soap.
- Keep vaccinations up to date.
- Avoid close contact with sick people.
- Drink clean water and eat food that is cooked and stored safely.
- Use mosquito nets and repellent in areas with dengue and malaria.
Common Myths About Fever
- “Fever damages the brain.” Fevers from infections rarely go above 41°C, and they do not damage the brain.
- “Feed a cold, starve a fever.” Drink fluids, and eat when you feel hungry.
- “Cold water brings the temperature down quickly.” It can cause shivering and a rebound rise.
Fever in Pregnancy
A fever during pregnancy needs a doctor’s attention, especially in the first three months. Paracetamol is the usual medicine for fever in pregnancy. Do not use ibuprofen or aspirin unless your doctor advises it. Call your doctor if your temperature reaches 38°C (100.4°F).
Fever After Vaccination
A mild fever within one or two days after a vaccine shows that your immune system is responding. Give fluids and rest. Paracetamol can ease discomfort. Call a doctor if the fever is above 39.4°C (103°F), lasts more than two days, or comes with a seizure, extreme sleepiness, or trouble breathing.
Simple Home Fever Kit
Keep these items at home so you can respond quickly:
- A digital thermometer
- Paracetamol in the right form for your family, such as tablets or syrup
- Oral rehydration salts
- A clean cloth for lukewarm sponging
- The phone number of your doctor and the nearest hospital
Frequently Asked Questions
Is fever a disease?
No. It is a sign that your body is fighting something.
Should I always treat a fever?
No. Treat it when it makes you uncomfortable or if a doctor advises it.
How long does a fever last?
Most viral fevers last two to three days. See a doctor if it lasts longer.
Can teething cause a high fever?
No. Teething can cause a slight rise, but a high fever needs a doctor.
Can I go to school or work with a fever?
Stay home until your temperature has been normal for 24 hours without medicine.
Disclaimer: This article is for information only. It is not medical advice. Please talk to your doctor about your own health.