Fever is one of the body's most important defence mechanisms against infection, yet it remains widely misunderstood. For Australian families, knowing what constitutes a fever, understanding when it becomes concerning, and recognising the difference between various measurement methods can mean the difference between unnecessary worry and appropriate medical intervention.
This comprehensive guide breaks down everything you need to know about fever temperatures according to Australian health guidelines, helping you respond confidently when illness strikes your household.
What Exactly Is a Fever?
A fever occurs when your body's internal thermostat, located in the hypothalamus, temporarily resets to a higher temperature. This isn't a malfunction—it's actually a purposeful response that helps your immune system fight infections more effectively. Many viruses and bacteria struggle to survive at elevated temperatures, making fever a valuable ally in recovery.
According to Australian health authorities, including the Royal Children's Hospital Melbourne and NSW Health, a fever is generally defined as a body temperature at or above 38°C (100.4°F). However, this threshold can vary slightly depending on the measurement method used and the individual's baseline temperature.
Normal body temperature typically ranges from 36.1°C to 37.2°C (97°F to 99°F), though individual variations are common. Temperatures between 37.2°C and 38°C are considered mildly elevated but not necessarily a fever.
Temperature Thresholds by Measurement Method
One of the most important factors affecting temperature readings is the measurement method. Different body sites yield different readings, and understanding these variations is crucial for accurate fever assessment.
Rectal Temperature (Most Accurate)
Rectal measurements provide the most accurate core body temperature reading. For this method:
- Normal range: 36.6°C to 38°C
- Fever threshold: 38°C or above
- High fever: 39.4°C or above
Oral Temperature
Oral readings are approximately 0.3°C to 0.6°C lower than rectal temperatures:
- Normal range: 35.5°C to 37.5°C
- Fever threshold: 37.8°C or above
- High fever: 39°C or above
Ear (Tympanic) Temperature
Ear thermometers measure infrared heat from the eardrum:
- Normal range: 35.8°C to 38°C
- Fever threshold: 38°C or above
- High fever: 39.2°C or above
Forehead (Temporal) Temperature
Non-contact infrared thermometers measure skin surface temperature:
- Normal range: 35.8°C to 37.6°C
- Fever threshold: 37.8°C or above
- High fever: 39°C or above
Underarm (Axillary) Temperature
Underarm readings are typically 0.5°C to 1°C lower than core temperature:
- Normal range: 34.7°C to 37.3°C
- Fever threshold: 37.3°C or above
- High fever: 38.7°C or above
Key Takeaway
Always use the same measurement method when tracking a fever over time. Switching between methods makes it difficult to assess whether temperatures are rising or falling.
Age-Specific Fever Considerations
The significance of a fever varies considerably with age. What might be a routine viral illness in a school-aged child could be a serious concern in a newborn.
Newborns (0-3 Months)
Any fever in a baby under three months requires immediate medical attention. Their immune systems are still developing, and fever can indicate serious bacterial infections. Contact your doctor or visit an emergency department if your newborn has a temperature of 38°C or above.
Infants (3-12 Months)
While fever is more common and often less concerning at this age, temperatures above 39°C warrant a doctor's consultation, especially if accompanied by other symptoms like lethargy, poor feeding, or rash.
Toddlers and Children (1-12 Years)
Children in this age group commonly develop fevers with viral infections. Focus less on the number and more on how your child looks and behaves. A child who is playing and alert with a fever of 39°C is often less concerning than a listless child with a temperature of 38.5°C.
Adults and Elderly
Adults generally tolerate fever well, but elderly individuals may have a diminished fever response. A temperature that seems "normal" in an older person could actually represent a significant infection.
Seek emergency care if fever is accompanied by: severe headache with stiff neck, difficulty breathing, persistent vomiting, rash that doesn't blanch, confusion or altered consciousness, or seizures.
Factors That Affect Body Temperature
Understanding natural temperature fluctuations helps prevent misinterpretation of readings:
- Time of day: Body temperature is typically lowest in early morning and highest in late afternoon, varying by up to 0.5°C
- Physical activity: Exercise can temporarily elevate temperature
- Menstrual cycle: Women's temperatures rise slightly after ovulation
- Hot weather: Extreme heat can cause mild elevation
- Recent food or drink: Hot or cold beverages can affect oral readings
- Clothing and bedding: Overdressing, especially in children, can raise temperature
Managing Fever at Home
Most fevers don't require aggressive treatment. The goal isn't necessarily to eliminate the fever but to keep the person comfortable:
- Ensure adequate fluid intake to prevent dehydration
- Dress in light, breathable clothing
- Keep room temperature comfortable (not cold)
- Use paracetamol or ibuprofen as directed for comfort (not routinely)
- Avoid cold baths or sponging, which can cause shivering
- Rest and monitor for changes
Remember, treating fever with medication doesn't speed recovery—it simply improves comfort. Australian guidelines emphasise that the decision to treat should be based on the person's overall comfort rather than achieving a specific temperature target.
When Temperature Monitoring Matters Most
Accurate temperature monitoring is particularly important in certain situations:
- Tracking illness progression in young children
- Monitoring individuals with compromised immune systems
- Post-surgical recovery periods
- During outbreaks of infectious diseases
- For individuals taking immunosuppressive medications
Having a reliable thermometer and understanding how to use it correctly ensures you're equipped to make informed decisions about your family's health.