Why Do We Get a Fever? How the Body Turns Up Its Thermostat

A fever happens when the immune system tells the brain to raise the body’s temperature set point. Immune cells release signaling molecules that lead to prostaglandin E2 acting on temperature-control neurons in the hypothalamus, and the body then heats itself by shivering and holding in warmth. A review in Nature Reviews Immunology describes the febrile range as about 1 to 4°C (2 to 7°F) above baseline.

So a fever is not the germs “cooking” you. It is a controlled response, and versions of it are found across vertebrates, including cold-blooded animals that cannot make their own heat.

What actually triggers a fever?

Fever starts when immune cells detect signs of infection or tissue damage. A 2015 review in Nature Reviews Immunology describes the chain of events. Immune cells such as macrophages sense molecules from microbes, for example parts of bacterial cell walls, and release signaling proteins called pyrogenic cytokines, including interleukin-1, interleukin-6 and tumor necrosis factor.

These signals lead to the production of prostaglandin E2, a small fatty molecule. Prostaglandin E2 acts on neurons in a region of the hypothalamus called the preoptic area, the brain’s main temperature control center. Those neurons then raise the temperature the body tries to hold.

Substances that cause fever are called pyrogens, from the Greek word for fire. Some come from microbes. Others, like the cytokines above, are made by the body itself.

Not every microbe sets off this alarm. The gut alone carries tens of trillions of bacteria that normally live alongside the immune system without causing a fever, as we explain in what the gut microbiome is.

Why do you get chills when a fever starts?

You feel cold because your body is suddenly below its new target temperature. Once the set point rises, a normal body temperature registers as too low, so the brain switches on heat-making and heat-saving responses.

The review describes these responses: blood vessels in the skin narrow so less heat escapes, brown fat burns energy to make heat, and muscles shiver. That is why someone with a rising fever can be shaking under blankets while their temperature climbs. When the fever breaks and the set point drops back, the opposite happens. The body is now too warm for its target, so skin vessels widen and sweating starts.

1. Immune cells detect infection and release cytokines.
2. Prostaglandin E2 is produced and reaches the hypothalamus.
3. The temperature set point rises, and you feel chills.
4. Shivering and narrowed skin blood vessels raise body temperature.
5. When the signals fade, the set point falls, and sweating cools the body.

Does a fever help fight infection?

The evidence suggests it often does, but most of it comes from animals and lab experiments. The Nature Reviews Immunology review collects several lines of evidence. In cell cultures, temperatures of 40 to 41°C (104 to 106°F) sharply reduced how fast poliovirus copied itself. In an older rabbit experiment, animals infected with a virus and given a fever-lowering drug died at a much higher rate than rabbits allowed to run a fever. Raised temperatures also make several kinds of immune cells work harder in lab and animal studies.

Human evidence is thinner and mostly observational. Studies that compare patients who did and did not receive fever-reducing drugs cannot fully separate the drug’s effect from how sick the patients were to begin with. The review also points out that fever is not always helpful. In severe sepsis or brain injury, very high temperatures are linked to worse outcomes.

This is a description of body science, not treatment guidance. Whether and how to treat a fever in a child or adult is a question for a doctor, nurse or pharmacist.

Do cold-blooded animals get fevers?

Yes, they do it with behavior instead of internal heat. Reptiles, fish and many invertebrates cannot raise their own temperature much, so when they are infected, many of them move to warmer places. Scientists call this behavioral fever.

The review describes experiments with desert iguanas. Infected lizards chose warmer spots, and when researchers prevented them from warming up, their survival dropped by about 75%. Similar heat-seeking after infection has been seen in fish and even leeches, and in some cases fever-reducing drugs blocked it. The review’s authors note that fever has been conserved in warm- and cold-blooded vertebrates for more than 600 million years, which suggests it is worth its high energy cost.

Body temperature shifts in other ways in the animal world too. Some mammals drop their temperature sharply to save energy, as we explain in how animals sleep, nap and enter torpor.

Is 98.6°F really normal body temperature?

It is a historical average, not a fixed number. The figure goes back to the German physician Carl Wunderlich, who in the mid-1800s recorded temperatures from about 25,000 patients in Leipzig and set 37°C (98.6°F) as the standard, with a range of 36.2 to 37.5°C.

A 2020 study in eLife by researchers at Stanford University analyzed 677,423 temperature measurements from three US data sets: Civil War veterans measured from 1860 to 1940, a national survey from 1971 to 1975, and Stanford patients seen from 2007 to 2017. Average temperature fell by about 0.03°C with each birth decade for men, and by a similar amount for women. The authors suggest that less chronic inflammation from infections, and changes in living conditions, may explain the drop. They also cite a 2017 study in The BMJ of more than 35,000 patients with a mean oral temperature of 36.6°C (97.9°F).

Temperature also varies within each person. MedlinePlus, from the US National Library of Medicine, notes that body temperature is usually highest in the evening and can rise with exercise, strong emotion, eating, heavy clothing and hot, humid rooms.

These thresholds describe how temperature is measured; they are not a guide to when to seek care. That depends on age, symptoms and medical history, so check with a clinician about your own situation.

How is a fever different from overheating?

In a fever, the brain has raised its target and the body works to reach it. In heat illness, the target has not changed, but the body cannot shed heat fast enough, for example during hard exercise on a hot day. The two can feel alike, but the mechanism is the opposite: fever is regulated, overheating is a failure of regulation.

Fever is one small, visible part of a much larger defense system, from first-line defenders to antibodies, which we explain in how the immune system works.

Bottom line

Fever is a regulated rise in body temperature. Immune signals lead to prostaglandin E2 resetting the hypothalamus, and the body shivers and conserves heat to reach the new target, usually 1 to 4°C higher. Evidence from animal and lab studies indicates fever helps fight some infections, though human evidence is mostly observational. And 98.6°F is a 19th-century average; modern measurements run slightly lower.

FAQ

Why do I sweat when a fever breaks?

When the immune signals fade, the brain lowers its temperature target. Your body is now warmer than the new target, so it widens skin blood vessels and sweats to shed heat.

Is a fever caused by the germs themselves?

Not directly. Molecules from microbes trigger immune cells, and the immune cells send the signals that reset the brain’s thermostat. The body raises its own temperature.

Has human body temperature really dropped?

A 2020 Stanford study of more than 677,000 measurements found average temperatures in the US have fallen by about 0.03°C per birth decade since the 1800s. The authors link the change to less chronic inflammation and better living conditions, though the cause is not proven.

Do animals other than mammals get fevers?

Yes. Many reptiles, fish and invertebrates seek warmer places when infected, a response called behavioral fever. In desert iguanas, preventing this lowered survival in experiments.