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Fever in Adults: Causes, Symptoms & Treatment

Fever in adults refers to a body temperature above 38 °C and is often a sign of infection. Learn about causes, symptoms, and treatment options.

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Fever in adults refers to a body temperature above 38 °C and is often a sign of infection. Learn about causes, symptoms, and treatment options.

What Is Fever in Adults?

Fever in adults is defined as a body temperature exceeding 38.0 °C (100.4 °F). The normal body temperature of a healthy adult ranges between 36.5 °C and 37.5 °C. Fever is not a disease in itself but a symptom indicating an underlying cause – most commonly an infection or an inflammatory response of the immune system. It is considered a natural defense mechanism of the body.

Causes

Fever can be triggered by a wide variety of factors:

Infections: The most common cause, including bacterial or viral infections such as the common cold, influenza, urinary tract infections, pneumonia, or COVID-19.
Inflammatory conditions: Autoimmune diseases such as rheumatoid arthritis can cause fever.
Medications: Certain drugs can cause fever as a side effect, known as drug fever.
Vaccinations: A mild and temporary fever can occur after receiving a vaccine.
Cancer: Certain malignancies, especially lymphomas, may be associated with fever.
Heat stroke: Overheating of the body due to extreme environmental temperatures.

Symptoms

In addition to an elevated body temperature, fever is often accompanied by other symptoms:

Chills and sweating
Headache and body aches
Fatigue and general weakness
Loss of appetite
Increased heart rate (tachycardia)
Difficulty concentrating or mild confusion in cases of very high fever

Classification by Temperature

Fever is classified based on the measured body temperature:

Low-grade fever: 37.5 °C – 38.0 °C
Mild fever: 38.1 °C – 38.5 °C
Moderate fever: 38.6 °C – 39.0 °C
High fever: 39.1 °C – 40.0 °C
Very high fever (hyperpyrexia): above 40.0 °C – a medical emergency

Diagnosis

Diagnosis begins with measuring body temperature – most accurately done rectally, but also possible orally, axillary (under the arm), or via the ear. The physician collects a thorough medical history and performs a physical examination. Depending on the suspected cause, additional investigations may follow:

Blood count and inflammatory markers (e.g., CRP, white blood cell count)
Urinalysis
Swabs or cultures (e.g., throat swab)
Imaging studies (X-ray, ultrasound) if specific conditions are suspected

Treatment

General Measures

For mild to moderate fever, supportive measures are recommended first:

Adequate fluid intake: Drink plenty of water or herbal tea to compensate for fluid loss through sweating.
Physical rest and bed rest
Warm clothing during chills; cooling measures (e.g., cool damp cloths on the calves) for very high fever

Medication

Fever-reducing medications (antipyretics) are used when fever exceeds 39.0 °C or the patient is significantly unwell. Common active substances include:

Paracetamol (acetaminophen) (well-tolerated, gentle on the stomach)
Ibuprofen (additionally anti-inflammatory)
Acetylsalicylic acid (aspirin) (not recommended for certain groups)

Treatment of the underlying cause is essential: bacterial infections are treated with antibiotics, while viral illnesses are typically managed symptomatically.

When to See a Doctor

Medical attention is urgently recommended in the following situations:

Fever above 39.5 °C that does not decrease
Fever lasting longer than 3 days
Severe accompanying symptoms such as stiff neck, skin rash, shortness of breath, or confusion
Fever in immunocompromised individuals or people with serious underlying conditions
Recurrent fever without an identifiable cause

References

World Health Organization (WHO): Fever – General Information. www.who.int
Fauci, A. et al.: Harrison's Principles of Internal Medicine, 21st Edition. McGraw-Hill, 2022.
National Institutes of Health (NIH) – MedlinePlus: Fever in Adults. www.medlineplus.gov

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