Understanding Fever: Normal Body Temperature, Thresholds and Common Causes
What Is Fever?
Fever is not a disease in its own right but a sign that the body is responding to something – most commonly an infection. In adults, a temperature measured orally, under the arm or in the ear from about 38.0 °C (100.4 °F) upwards is usually considered fever. The range 37.5–38.0 °C is often called low-grade fever or slightly elevated temperature.
"Fever is the body's way of protecting itself, but it is important to recognise warning signs."
What matters is not just the number on the thermometer but the overall condition: whether the person can eat and drink, is very thirsty, passing less urine, has breathing difficulties, is drowsy or has an unusual rash – especially in young children and older adults.
What Is Normal Body Temperature?
Many people think of 37.0 °C (98.6 °F) as "normal", but that is only an average. Normal body temperature in a healthy person usually ranges between 36.0–37.5 °C (96.8–99.5 °F) and can vary from person to person.
Temperature also changes throughout the day, depending on activity, age, hormones and measurement site. Evening temperatures are typically higher than morning readings. In children, normal temperature may be slightly higher than in adults.
| Temperature | Classification |
|---|---|
| Below 37.5 °C | Usually within normal range (varies by individual) |
| 37.5–38.0 °C | Low-grade fever / slightly elevated |
| ≥ 38.0 °C | Fever |
| ≥ 39.0–40.0 °C | High fever, requires close monitoring |
Why Does the Body Develop Fever?
When viruses, bacteria or other pathogens enter the body, the immune system responds by releasing signalling molecules (pyrogens). These act on the temperature-regulating centre in the brain (hypothalamus), resetting the body's thermostat to a higher level – similar to turning up the heating.
The body then tries to raise its temperature: blood vessels in the skin constrict, you feel cold, get goosebumps and may shiver to generate heat. Once the temperature reaches its new set point, you feel hot, flushed and may sweat profusely.
Higher temperatures can slow down the replication of some viruses and bacteria and may help certain immune cells work more effectively. However, prolonged high fever can lead to dehydration, exhaustion, and can be more dangerous in young children, older adults and people with chronic conditions.
Common Causes of Fever
- Respiratory viral infections: flu, COVID-19, sore throat, bronchitis
- Gastrointestinal viral infections: stomach pain, diarrhoea, vomiting
- Bacterial infections: middle ear infection, pneumonia, urinary tract infection
- Post-vaccination fever: usually mild and lasts 1–2 days
If you have recently travelled to a region with tropical infections, inform your doctor and do not self-treat for extended periods.
When to Seek Medical Attention
Contact a healthcare provider if:
- Fever persists for more than 3 days
- Temperature exceeds 39.5 °C (103 °F)
- There are warning signs such as difficulty breathing, confusion, persistent vomiting, or rash
- The person is a young infant, elderly, or has chronic health conditions