A fever is more than simply becoming too warm. It is a controlled change in the body’s internal temperature setting, usually triggered by the immune system in response to an infection or another form of inflammation.
When you develop a fever, your brain temporarily raises the temperature your body tries to maintain. Your body then works to reach that higher target by conserving heat and producing more of it. That is why a fever can make you feel cold and cause chills even while your measured temperature is rising.
Understanding this distinction explains much of what happens during a fever, from shivering and sweating to why fever-reducing medicines can make you feel better.
What actually causes a fever?
Your body normally keeps its internal temperature within a relatively narrow range. A part of the brain called the hypothalamus acts as a temperature-regulating center. It receives information about body temperature and coordinates responses that either conserve heat or release it.
During many infections, immune cells detect invading organisms and release signaling molecules that promote inflammation. Some of these signals ultimately cause the body to produce substances called prostaglandins. One important prostaglandin, PGE2, acts on the hypothalamus and raises the body’s temperature set point.
The result is a fever.
This is different from hyperthermia, in which body temperature rises because the body is gaining or producing more heat than it can effectively lose—for example, during severe overheating. In hyperthermia, the brain’s temperature set point has not been deliberately raised. In a fever, the body is actively defending a higher temperature.
Why do you get chills when you’re already hot?
The change in the temperature set point happens before the rest of the body has necessarily become warmer.
Suppose your temperature is 98.6°F and the hypothalamus raises the target temperature several degrees. Your existing temperature now feels too low relative to the new target. The nervous system responds as though the body is cold.
Blood vessels near the skin can narrow, reducing heat loss. You may seek warmth, get goosebumps, and begin to shiver. Shivering rapidly contracts muscles, generating additional heat. Together, these responses raise the body’s temperature toward the new set point.
That is why someone with a rising fever may be bundled under blankets and feel intensely cold despite having an elevated thermometer reading.
Once the body reaches the new set point, the chills generally subside. Later, when the fever breaks and the hypothalamus returns the set point toward normal, the body suddenly has excess heat relative to its new target. Blood vessels in the skin widen and sweating increases, allowing heat to escape. This is the familiar sweating that can accompany a falling fever.
Why does the immune system raise body temperature?
A fever is part of the body’s coordinated response to illness, rather than an accidental consequence of being sick.
Higher temperatures can affect the behavior of immune cells and the growth or survival of some microorganisms. Fever also occurs alongside other inflammatory responses that help the body detect and respond to infection.
However, a fever is not necessarily beneficial in every circumstance, and higher is not always better. The body’s response is regulated, and very high temperatures can become harmful. The physiological costs of maintaining a higher temperature include increased metabolic activity and greater demands on the body’s systems.
A fever therefore represents a regulated immune response, not a simple mechanism for “killing germs with heat.”
Why does your temperature rise during an infection?
Many infections trigger the release of inflammatory signals known as pyrogens. The term pyrogen refers to a substance that can cause or promote fever.
Some pyrogens come from the body’s own immune response. Others can originate from infectious organisms or their components. These signals contribute to the production of prostaglandins in the brain, particularly PGE2, which influences the hypothalamus.
The chain of events can be summarized as:
Infection or inflammation → immune signaling → prostaglandin production → higher hypothalamic set point → heat conservation and production → fever
Once the underlying inflammatory signal decreases, the set point can return toward normal. The body then releases the excess heat through increased blood flow to the skin and sweating.
How is a fever different from simply feeling hot?
Feeling hot does not necessarily mean you have a fever.
Your body temperature can rise temporarily after exercise, in a hot environment, or under other normal circumstances. In these situations, the body generally responds by trying to lower its temperature, especially through sweating and increased blood flow to the skin.
A fever works in the opposite direction at first. The hypothalamus has intentionally raised the temperature target, so the body initially tries to increase and retain heat.
This distinction is important because someone with a fever may feel cold during the temperature’s rise and hot or sweaty as it falls.
What temperature counts as a fever?
For adults, a temperature of 100.4°F (38°C) or higher is commonly used as the threshold for fever, although the exact reading can vary with the measurement method, time of day, and individual circumstances.
Body temperature also naturally fluctuates. It tends to be lower at some times of day and higher at others, so a single measurement should be interpreted in context.
The location and method of measurement matter as well. Oral, rectal, ear, and underarm measurements do not always produce identical readings. For that reason, it is useful to follow the thermometer’s instructions and use a consistent method when monitoring a person’s temperature.
Why do fever-reducing medicines lower your temperature?
Medications such as acetaminophen and ibuprofen can reduce fever partly by interfering with the pathways that produce prostaglandins.
When the hypothalamic set point moves back toward normal, the body no longer needs to conserve and generate as much heat. It can instead begin releasing heat. Someone who was shivering may stop feeling cold and may begin to sweat as the temperature falls.
These medicines can make a person more comfortable, but reducing a fever does not necessarily eliminate the infection that caused it. The underlying illness still determines how long the fever lasts and what other symptoms occur.
Medication choice and dosing also depend on age, health conditions, other medicines, and the specific product. In children, aspirin should not be used for treating fever unless a clinician specifically recommends it because of its association with Reye syndrome in children and teenagers recovering from certain viral illnesses.
Is every fever caused by an infection?
No. Infections are a common cause, but fever can also occur with noninfectious inflammation, certain medications, immune-system disorders, some cancers, and other medical conditions.
The mechanism can still involve inflammatory signaling and changes in the hypothalamic temperature set point.
This is another reason fever should be understood as a physiological response, rather than a disease itself. The temperature rise is a symptom or sign of an underlying process.
When is a fever a reason to seek medical care?
The temperature itself is only one part of assessing an illness. Age, duration, other symptoms, underlying health conditions, and how severely the person appears to be affected all matter.
Prompt medical evaluation is particularly important when a fever occurs with symptoms such as difficulty breathing, severe or persistent confusion, a seizure, a stiff neck with severe illness, severe dehydration, or a rapidly worsening condition. Infants younger than 3 months with a temperature of 100.4°F (38°C) or higher should receive prompt medical evaluation.
For older children and adults, a fever that persists, repeatedly returns, or occurs with concerning symptoms also warrants medical advice.
The key point is that a fever is usually the result of the body’s temperature-control system being deliberately reset upward. Chills are the body’s attempt to reach that new set point; sweating often signals that the set point has fallen again. The temperature rise is therefore not simply the body “getting hot.” It is a coordinated physiological response involving the immune system, inflammatory signals, the brain, blood vessels, muscles, and mechanisms of heat loss.
