Why Does Infection Cause Fever?


A fever is a temporary increase in the body’s temperature in response to a disease or illness.

Medically, it is defined as having a temperature above the normal range due to an increase in the body’s temperature set-point. There is not a single agreed-upon upper limit for normal temperature with sources using values between 37.5 and 38.3 °C (99.5 and 100.9 °F).

The increase in set-point triggers increased muscle contraction and causes a feeling of cold. This results in greater heat production and efforts to conserve heat. When the set-point temperature returns to normal, a person feels hot, becomes flushed, and may begin to sweat.


Body temperature is regulated by the hypothalamus. A trigger of the fever, called a pyrogen, causes a release of prostaglandin E2 (PGE2). PGE2 then in turn acts on the hypothalamus, which generates a systemic response back to the rest of the body, causing heat-creating effects to match a new temperature level.

The hypothalamus works much like a thermostat. When the set point is raised, the body increases its temperature through both active generation of heat and retention of heat.

Peripheral vasoconstriction both reduces heat loss through the skin and causes the person to feel cold. If these measures are insufficient to make the blood temperature in the brain match the new set point in the hypothalamus, then shivering begins in order to use muscle movements to produce more heat.

When the hypothalamic set point moves back to baseline either spontaneously or with medication, the reverse of these processes (vasodilation, end of shivering and nonshivering heat production) and sweating are used to cool the body to the new, lower setting.

Pyrogens are cytokines, molecules that are a part of the immune system. They are produced by activated immune cells and cause the increase in the thermoregulatory set point in the hypothalamus. Major endogenous pyrogens are interleukin 1 (α and β) and interleukin 6 (IL-6).

Minor endogenous pyrogens include interleukin-8, tumor necrosis factor-β, macrophage inflammatory protein-α and macrophage inflammatory protein-β as well as interferon-α, interferon-β, and interferon-γ. Tumor necrosis factor-α also acts as a pyrogen. It is mediated by interleukin 1 (IL-1) release.

These cytokine factors are released into general circulation, where they migrate to the circumventricular organs of the brain due to easier absorption caused by the blood–brain barrier’s reduced filtration action there. The cytokine factors then bind with endothelial receptors on vessel walls, or interact with local microglia cells. When these cytokine factors bind, the arachidonic acid pathway is then activated.

Clearing Up Myths about Fevers

1. Understand healthy fevers. You might be surprised to learn that fever is a symptom, not an illness. It’s your body’s way of defending itself against infection. You feel hot because blood vessels are expanding to bring the excess heat up to the surface of your skin.

2. Find your baseline. Temperature readings of 98.6F are an average figure based on the fact that your temperature fluctuates throughout the day, usually peaking in late afternoon. Measure yourself a few times on a day when you feel well to find your normal range.

3. Monitor recurring fevers. Fevers that come and go can feel like they’re continuous. Medical tests will rule out more serious causes if you’re experiencing 6 fevers or more a year.

4. Get out of bed. Did your parents order you to stay under the covers? As long as your condition isn’t contagious, you can probably go to work. Just avoid strenuous activity.

5. Call your doctor. Immediate medical care is required for infants with rectal temperatures of 100.4F or higher, or any child with a fever above 104F. Most adults will need to see a doctor for temperatures starting at 105F or additional symptoms including a stiff neck, severe stomach pain, or skin rashes.

Taking Your Temperature

1. Pick your method. You can take your temperature almost anywhere on your body. Rectal thermometers are recommended for children under age 3. Older children and adults will usually get the most precise and convenient reading from an oral thermometer.

2. Watch your timing. Many factors can affect your body temperature. That includes eating, intense exercise, or a hot shower, so wait at least 30 minutes to ensure an accurate reading.

3. Follow instructions. Read the directions that come with your thermometer. For an oral reading, place the instrument under your tongue, and keep your mouth closed for the recommended amount of time, which is usually about 3 minutes.

4. Take care of your thermometer. For good hygiene, clean your thermometer after each use with cool water and soap. If you’re using a digital model, check the batteries, especially if it’s been stored away for a long time.

Treating Your Fever

1. Visit the drugstore. It’s usually safe to let a mild fever run its course, but you can treat higher temperatures and relieve discomfort with over-the-counter medications like acetaminophen or ibuprofen. Your doctor may also prescribe an antibiotic for the underlying infection.

2. Lower the thermostat. Keeping your house cool will help you to rest and sleep more comfortably. Keep a light blanket handy if you feel chilly.

3. Dress in layers. Multiple layers are better than heavy clothing or bedding that could make you sweat. That kind of rapid heat loss actually makes a fever worse.

4. Drink liquids. You might not feel like eating much when you’re sick, but it’s important to stay hydrated. Sip water, low sodium soup, or other beverages.

5. Sponge off. Taking a sponge bath while sitting in shallow water provides additional quick relief. Stick to warm water to prevent chills.

Keep your cool about fevers. Call your doctor if you or your child have a dangerously high temperature or show other warning signs such as seizures or being unresponsive.

Otherwise, fever symptoms are usually a sign that your immune system is working properly and protecting you from illness.

Kluge, Matthew J. (2015)
Fever: Its Biology, Evolution, and Function
Princeton University Press. p. 57. ISBN 9781400869831

Last Updated on November 15, 2022