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When Do You Have a Fever? Thresholds and Tips

A fever starts at a body temperature of 38.0 °C (100.4 °F). Temperatures between 37.5 °C and 38.0 °C are considered elevated. Learn when a fever becomes dangerous.

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Things worth knowing about "When Do You Have a Fever"

A fever starts at a body temperature of 38.0 °C (100.4 °F). Temperatures between 37.5 °C and 38.0 °C are considered elevated. Learn when a fever becomes dangerous.

When Is It Considered a Fever?

A fever is defined as an elevated core body temperature that occurs as a response of the immune system to infections or other medical conditions. The thresholds vary slightly depending on the measurement site, but the following general classification applies:

Normal body temperature: 36.0 °C – 37.4 °C (97.0 °F – 99.3 °F)
Elevated temperature (low-grade fever): 37.5 °C – 37.9 °C (99.5 °F – 100.2 °F)
Fever: from 38.0 °C (100.4 °F)
High fever: from 39.0 °C (102.2 °F)
Very high fever (hyperpyrexia): from 41.0 °C (105.8 °F) – medical emergency

Measurement Sites and Temperature Differences

The measured body temperature depends strongly on the measurement site. Slightly different thresholds apply depending on where you measure:

Rectal (rectum): Most accurate method, fever from 38.0 °C (100.4 °F)
Oral (mouth): Fever from 37.8 °C (100.0 °F)
Axillary (armpit): Fever from 37.4 °C – 37.5 °C (99.3 °F) – less accurate
Tympanic (ear): Fever from 38.0 °C (100.4 °F)
Forehead (infrared): Fever from 38.0 °C (100.4 °F) – device dependent

Causes of Fever

Fever is in most cases a sign that the immune system is actively fighting pathogens. Common causes include:

Bacterial or viral infections (e.g., influenza, common cold, COVID-19)
Urinary tract infections
Inflammatory processes in the body
Autoimmune diseases
Vaccine reactions
Overheating (heat stroke)

Symptoms Associated with Fever

Fever often occurs together with other complaints, including:

Chills and shivering
Headaches
Body aches and fatigue
Sweating
Loss of appetite
Increased heart rate

When to See a Doctor?

Medical attention should be sought promptly in the following situations:

Fever above 39.5 °C (103.1 °F) that does not respond to fever-reducing medication
Fever in infants under 3 months of age from 38.0 °C (100.4 °F)
Fever lasting longer than 3 days
Accompanying symptoms such as neck stiffness, skin rash, altered consciousness, or difficulty breathing
Fever following travel to tropical regions

Reducing a Fever – What Helps?

Mild fever does not need to be reduced immediately, as it helps the body fight pathogens. For severe discomfort or high fever, the following measures can help:

Drink plenty of fluids (water, herbal tea)
Rest and bed rest
Fever-reducing medications such as paracetamol or ibuprofen (follow package instructions or medical advice)
Cool compresses on the calves for very high fever

References

World Health Organization (WHO): Fever Management. www.who.int
Robert Koch Institut (RKI): Fever – General Information. www.rki.de
Kasper, D. L. et al. (2018): Harrison's Principles of Internal Medicine, 20th Edition. McGraw-Hill Education.

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