What is a fever and what are its causes?

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Fever: Definition, Symptoms, Causes, and Management

Fever is not a disease but a type of immune response indicating that the body’s immune system is activated. It is a sign that the body has started producing white blood cells to fight an underlying condition. Normal body temperature in adults ranges between 36.1°C and 37.6°C (97°F–99.7°F). In children, these values may be slightly higher. Additionally, temperature readings vary depending on the site of measurement, and body temperature naturally fluctuates throughout the day. It tends to peak in the afternoon and evening and decreases in the early morning. Daily variations can reach up to 1°C (1.8°F).

What is Fever?

Fever can occur due to many conditions, including bacterial or viral infections. It results from an immune response that affects the hypothalamus, temporarily raising body temperature. Fever is a sign that the immune system is fighting something. It can be measured in the armpit, mouth, ear, or rectum, and values may differ slightly depending on the measurement site. Measurements taken immediately after consuming hot or cold food or drinks may be inaccurate.

Symptoms of Fever

The most common complaints in people with fever are chills and feeling cold. Peripheral blood vessels constrict, causing coldness in extremities such as hands and feet. Heart rate and metabolism increase, with a 10–12% rise in metabolic rate at temperatures above 37°C. The body’s need for fluids and calories also increases. The cardiovascular and respiratory systems are under additional strain, leading to fatigue and weakness.

Common symptoms of fever include:

  • Chills

  • Rapid heartbeat

  • Sweating

  • Headache

  • Muscle aches

  • Loss of appetite

  • Weakness and fatigue

  • Increased sensitivity to pain

  • Numbness

  • Difficulty concentrating

Seek medical attention immediately if fever is accompanied by:

  • Sensitivity to light

  • Stiff neck and pain

  • Shortness of breath

  • Nausea and vomiting

  • Severe thirst

  • Dizziness

  • Abdominal pain

  • Muscle cramps

  • Confusion or delirium

Additionally, fever accompanied by painful, dark, or foul-smelling urine may indicate a serious condition and requires prompt medical evaluation.

Causes of Fever

In most cases, mild fever caused by infection resolves on its own within 1–3 days, responds to antipyretics, and is part of the healing process. However, persistent or recurrent fever, even if low-grade, may indicate a serious infection and should not be ignored. Fever is classified by temperature as:

  • Moderate: up to 38°C (100.4°F)

  • High: above 39°C (102.2°F)

Fever can also be classified by duration:

  • Acute: sudden onset, lasting <7 days

  • Subacute: lasting 7–14 days

  • Chronic: lasting >14 days

Common causes of fever in adults include:

  • Respiratory infections

  • Bacterial or viral infections (ear, lung, skin, throat, urinary tract, kidney, etc.)

  • Fungal infections

  • Food poisoning

  • Heatstroke or sunburn

  • Inflammatory conditions (e.g., rheumatoid arthritis)

  • Infections from insect or tick bites (e.g., Lyme disease, malaria, Crimean-Congo hemorrhagic fever)

More serious causes may include hyperthyroidism, autoimmune diseases, certain cancers, and tumors. Fever can also occur during substance withdrawal or alcohol cessation, or as a side effect of certain medications, vaccines (e.g., rabies, tetanus, pneumonia), steroids, chemotherapy, radiotherapy, or post-organ transplant therapy.

People with pre-existing chronic illnesses are at higher risk, and fever in these individuals should be promptly evaluated. Special caution is required for those with:

  • Asthma

  • Rheumatoid arthritis

  • Diabetes

  • Autoimmune diseases (Crohn’s disease, lupus, etc.)

  • Cardiovascular diseases

  • Anemia

  • Liver or kidney disorders

  • Cerebral palsy

  • Multiple sclerosis

  • Chronic lung disease

  • HIV/AIDS

Managing Fever

Since fever is a symptom rather than a disease, treatment depends on its underlying cause. In infections, treating the infection often reduces fever. Nevertheless, it is important to manage fever to prevent organ damage or complications.

Fever can be reduced with:

  • Antipyretics such as paracetamol, ibuprofen, or salicylates

  • Cold compresses

For mild fever, adequate nutrition, light exercise, and proper sleep support the immune system and can shorten the course of infection and reduce fever.

Hydration is crucial: water, fruit juices, broths, and bone soups provide fluids and calories without overloading the stomach. Adults may also benefit from warm baths and maintaining a moderate room temperature to alleviate fever symptoms.

When to See a Doctor

Seek immediate medical attention if:

  • Adults have a temperature above 39°C (102.2°F)

  • Children under 3 years have a temperature above 38°C (100.4°F)

  • Fever in children is accompanied by cough, diarrhea, vomiting, or rash

  • Fever occurs during heatstroke

  • Fever persists despite antipyretics