What happens to our body during a fever? The science of your immune response

September 28, 2025 •

2 min read

When your body detects an invasion by a pathogen, a complex physiological cascade is triggered, resetting your internal thermostat and causing a rise in temperature. This is what happens to our body during a fever, a natural and crucial part of your immune system's defense strategy.

Quick Summary

A fever is a controlled increase in the body's temperature, orchestrated by the brain's hypothalamus in response to infectious or inflammatory triggers. The raised temperature creates a less hospitable environment for pathogens while boosting the efficiency of immune cells to fight the underlying illness.

Key Points

Hypothalamus Reset: The brain's thermostat resets to a higher temperature in response to pyrogens from the immune system, causing a fever [1, 2].
Chills and Shivering: These symptoms are your body's way of generating heat to meet the new, elevated temperature set point [1, 3].
Immune System Boost: A higher body temperature increases the activity of white blood cells and other immune defenses, helping to fight infection more effectively [2].
Systemic Strain: A fever increases metabolism, heart rate, and fluid loss, leading to fatigue, muscle aches, and dehydration [4].
Fever vs. Hyperthermia: A fever is a controlled, regulated process, whereas hyperthermia is an uncontrolled temperature rise due to the body's inability to cool itself [1].
Fever Resolution: As the infection is cleared, the hypothalamus returns to its normal setting, and the body cools itself by sweating [1, 2].

The Thermostat Effect: How Your Brain Controls the Heat

The hypothalamus, located in your brain, acts as your body's thermostat, regulating core temperature [1]. During an infection, the immune system releases pyrogens that signal the hypothalamus to raise its temperature set point [1, 2]. This change triggers actions like shivering to generate heat and blood vessel constriction to conserve it, causing the body's temperature to rise to the new setting and leading to chills [1, 3].

The Immune Response in Overdrive

A fever is a deliberate immune strategy. The higher temperature hinders pathogen replication and boosts the activity of immune cells like white blood cells and the production of antiviral proteins such as interferons [2].

Benefits of a Fever

Inhibits Pathogen Growth: Higher temperatures make it difficult for many pathogens to multiply [2].
Enhances Immune Cell Function: Immune cells become more active and efficient [2].
Boosts Interferon Production: Antiviral protein creation increases [2].
Speeds up Chemical Reactions: Metabolic rate rises, aiding recovery processes [2, 4].

Systemic Effects: How Fever Impacts the Body's Systems

A fever puts stress on the body, causing fatigue, aches, and general discomfort [4]. The increased metabolic rate demands more oxygen, increasing heart and respiratory rates [4]. Dehydration is also a concern due to increased sweating [4].

Impact on the Cardiovascular and Renal Systems

Cardiovascular Strain: Elevated heart rate can be taxing, especially for those with heart conditions [4].
Dehydration Risk: Increased fluid loss can impact kidney function [4].
Acute Kidney Injury (AKI): Severe fevers may increase the risk of AKI [4].

Fever vs. Hyperthermia: A Critical Distinction

A fever is a controlled, regulated temperature increase, while hyperthermia is an uncontrolled rise [1]. Hyperthermia occurs when the body's cooling mechanisms are overwhelmed, often by external factors like heat or drugs, and can lead to dangerous temperatures and organ damage [1]. You can find more details in the provided source: Physiology, Fever [1].

Comparison Table: Low-Grade vs. High-Grade Fever


Feature	Low-Grade Fever (e.g., 99.1°F–100.4°F)	High-Grade Fever (e.g., >103°F)
Immune Purpose	Mild stimulation of immune response.	More aggressive response to potential serious infection.
Common Causes	Minor infections, stress.	Influenza, pneumonia, severe bacterial infections.
Key Symptoms	Mild fatigue, minor aches.	Severe chills, muscle aches, confusion.
Typical Action	Rest, fluids.	Monitoring, potential medication, seek medical advice if severe.
Associated Risk	Generally low.	Increased risk of complications.

The Resolution Phase: How Fever Ends

As the infection clears and pyrogen levels drop, the hypothalamus resets to the normal temperature [1]. The body cools down by sweating and dilating blood vessels in the skin [1, 2]. This process brings the body back to its baseline temperature, allowing for recovery and tissue repair [2, 4].

Conclusion: Understanding the Body's Defense Mechanism

Understanding what happens to our body during a fever reveals it as a sophisticated immune response rather than just a symptom [1, 2, 4]. This controlled temperature increase helps fight infection and supports the immune system [2]. While often manageable at home, it's crucial to recognize when a fever is severe and seek medical attention [5].

Frequently Asked Questions

A fever is generally defined as a body temperature of 100.4°F (38°C) or higher. This is a common benchmark, though it can vary slightly depending on measurement method.

Chills occur because your brain has raised your body's target temperature. Your body feels cold compared to this new set point and shivers to generate heat and raise your temperature [1, 3].

Typically, a fever is a helpful immune response that inhibits pathogens and boosts immune function [2]. However, very high or prolonged fevers can be harmful and may require medical care [4, 5].

Feeling tired and achy is due to the increased metabolic activity needed to maintain the fever and the release of inflammatory chemicals by your immune system [4].

Managing a fever naturally involves staying hydrated, resting, and wearing light clothing. Lukewarm baths or cool compresses can also offer some relief [4, 5].

Exercising with a fever is not recommended. It adds stress to your cardiovascular system, which is already working harder [4]. Rest is essential for recovery.

Consult a doctor if your fever is very high (over 103°F in adults), lasts more than a few days, or is accompanied by severe symptoms like confusion, stiff neck, severe headache, or difficulty breathing [5].