For most of human history, an unusually high body temperature was a sign of the supernatural. Fevers were sinister but common, unnatural but real. And without a theoretical underpinning by which to understand them, fevers long seemed to be as nebulous as they were deadly.

Ancient Romans had at least three temples dedicated to worshipping a god of fever. According to a 1918 issue of The Classical Weekly, Romans would leave amulets in these structures of febris, hoping to placate the deities who made them sick.

In the Middle Ages, fever treatments included incantations, elixirs, charms, and exorcisms. Avicenna, the influential Persian scholar, described the condition as “extraneous heat, kindled in the heart, from which it is diffused to the whole body through the arteries and veins.” In the centuries that followed, fevers retained an air of mysticism.

“It was generally agreed by sixteenth-century writers on fever, as by their predecessors, that the nature of fever lay in the ‘heat contrary to nature,’ or preternatural heat experienced by the patient,” wrote Iain Lonie in a 1981 paper for the Wellcome Trust. “Almost all of them regarded this heat as a distinct kind or genus of heat, contrasted with other kinds.”

A fever, then, wasn’t seen as an extension of a person’s body heat, but as something else entirely. Fevers were mysterious largely because people didn’t understand they were a symptom rather than a disease in their own right—but also because of their seemingly paradoxical qualities. A person who is febrile feels hot to the touch, but experiences bouts of shivering cold. Someone with a very high temperature can get well without intervention, whereas the condition of a person with milder fever can seem to suddenly deteriorate. “Fever is a mighty engine which nature brings into the world to the conquest of her enemies,” wrote the English physician Thomas Sydenham in the 1660s.

Today, fevers are common as ever, but often linked to less serious diseases. In many parts of the world, vaccination has eradicated some of the deadliest illnesses that can cause fevers. In an otherwise healthy person, a fever runs its course in a few days and then a person gets better. We have widely available medicines, like Tylenol, to treat discomfort associated with fevers. And yet they have retained much of their peculiar power.

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“People don’t like fever,” said Howard Markel, the director of the Center for the History of Medicine and a professor of pediatrics and communicable diseases at the University of Michigan. “They just don’t, and for good reason.”

A person with a fever, after all, can feel very sick—can actually be very sick. But the larger attitude toward fevers is one that Markel says is socially and culturally mediated through generations of misinformation and actual suffering. “We’re culturally raised to be wary of fever based on older information—some of it very pertinent to this day, some of it not so pertinent,” he said. “I don't want to call them all urban legends or myths, but these are cultural practices that spread just as widely as the infectious diseases that are behind them.”

The phenomenon known as “fever phobia,” an exaggerated fear of fever, is found among parents of all socioeconomic classes. One study found the majority of parents are unduly worried about low-grade fevers; another reported that most caregivers were “very worried” about the potential harm of fever in their children.

This fear is, in some ways, understandable. A fever—a temperature at or above 100.4 degrees Fahrenheit—in an newborn’s first month of life is a medical emergency. Infants that young are rushed to the emergency room for spinal taps and urine cultures to rule out bacterial infection that can be deadly. They are hospitalized and given IV antibiotics even before doctors can determine whether a baby is gravely ill. Hospitalization for this reason is quite common; serious problems are not. Babies can also suffer from febrile seizures, convulsions and loss of consciousness prompted by high body temperature. Such episodes affect one in 25 children, according to the National Institute of Neurological Disorders and Stroke. Although most kids outgrow febrile seizures without any lasting health problems, they can be frightening for caregivers.  

Fevers aren’t always a sign of a passing virus or some other benign illness. Among the the more perplexing conditions for doctors are persistent fevers that seem to have no cause, or fevers of unknown origin. Startlingly, many physicians still don't have a clear understanding of how to manage fevers, despite them being among the most common health problems. A 2001 study found a significant number of doctors demonstrated a “serious lack of knowledge” of the nature, dangers, and management of fever in children.

Avicenna, it should be pointed out, did not have it right about fevers, either. Physiologically, they are not “kindled in the heart,” but triggered by a reaction in the hypothalamus, a bundle of tiny nuclei about the weight of a penny that sits just above the brainstem. “Going back to Hippocrates, people—doctors and their patients—had the conception that fever was the disease itself rather than, say, caused by salmonella, or influenza, or some microbial organism,” Markel told me. “Nobody knew about microbes, so fever was considered a disease.”

In those days, most people weren’t as concerned with where a fever came from as they were with what might happen to you if you got one. “Very often you tended to have infectious diseases that killed you,” Markel said. “That’s one reason why fever would be terrifying.”

“If you have a high fever and it’s before the era of antibiotics, you’ll be sick as hell,” he continued. “With pneumonia, which was extremely common, you could have a really high fever of 104 or 105. People died of it. That’s scary stuff. And your body either fights it off or it doesn’t. Sometimes, with diseases like bacterial pneumonia, you’d be sweating, your fever would be high, and you’d either resolve it or you’d die. These were very dramatic moments.”

Evidence of these moments was everywhere, culturally. In the 18th and 19th centuries, newspaper pages were cluttered with ads for tonics promising to “cure fever.” And because “fever” was a euphemism for a number of serious illnesses, stories of people who “died from fever” were all over obituary pages.

“Back then, people said, ‘Fever came to town,’ and ‘This year’s fever had a lot of red rash.’ We now know that meant measles,” said David Morens, a senior scientific advisor at the National Institute of Allergy and Infectious Diseases. “When they said, ‘Last year fever came to town and it had a lot of vesicular rash,’ we now know that was smallpox. They didn’t understand fever in its different forms as indicating any distinct disease that was different from any other disease.”

Which is why generations of doctors tried to treat fevers themselves, not their underlying causes, often with gruesome outcomes. “Particularly with febrile diseases, one of the ideas was that you had unbalanced humors,” Morens said. “The idea was that bad humors were causing the disease, or at least the fever, and you had to get rid of the poison.” In the 1700s, patients with febrile diseases were bled, sometimes to death, in an attempt to get rid of toxicity. Other treatments included medicines that made a person vomit, sweat, or have diarrhea.

“The idea was the fever will go away if you get out all these bad things through all the various pores and orifices,” Morens said. “At the same time, people still believed that fever was caused by the pounding of the heart generating heat—which was viewed as a substance, not a property. They observed that when you had a disease, your heart rate goes up, but they had it backwards.”

The Italian physician Francisco Torti’s drawing of a fever tree, an attempt to connect various febrile illnesses, first published in 1712. (Wellcome Images)

Even into the 20th century, as people began to understand that fever was the symptom and not the disease, fever was explicitly connected to how illnesses were described: yellow fever, typhoid fever, scarlet fever.

Fever remained the focal point. It was a cause of death, right there in the name of the most terrible diseases you could get.

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The technology still used to assess a fever, the thermometer, reinforces a view of febrile illnesses that isn’t right. Thermometers allow for a level of precision—the difference between, say, 101.2 degrees and 101.7 degrees—that does little to actually illuminate how sick a person is. At the same time, thermometers have become an icon of modern medicine by being among the most common medical tools in a person’s home. Which means one of the key devices people associate with differentiating between sickness and wellness is ultimately misleading.

“A thermometer is, by its very nature, a crude measurement,” Morens said. “Temperature measurement is affected by the ambient temperature outside, it’s affected by daily variations. Different people have different ranges they go through at different times. It’s an imprecise measurement. I don’t think anybody who is a medical person or a scientist has ever had the idea that there’s any precision in measuring a temperature.”

This hasn’t stopped the baby-industrial complex from capitalizing on fever phobia. Devices like TempTraq, a wearable thermometer that transmits a baby’s temperature to a parent’s smartphone via a wireless signal, promises to monitor a child’s temperature continuously. There’s also, as Markel put it, a pharmaceutical industry that “makes a fortune” off fever reducers.

Today, doctors don’t treat fevers as much as they attempt to treat a fever’s underlying cause. “We understand that if you get rid of the bug you get rid of the fever,” Morens said. “We see fever as a sign of an illness but treating the fever is largely meaningless. There are thousands and thousands of infectious diseases that, under some circumstances at least, cause fevers. That’s what we know now.”

But philosophies on fevers are evolving. Doctors still debate the proper treatment for newborns with fevers—should a 3-month-old get the same aggressive workup, including lumbar puncture and IV antibiotics, as a 3-day-old? The answer, as with so many questions in medicine is: It depends.

As recently as the 1970s, doctors still wondered if inciting fever might help kill off an actual infection. There’s little scientific evidence that a fever can stop a virus, but that idea has persisted—sort of like the bromide “feed a cold, starve a fever,” which is also without scientific proof.

When Morens was in medical school, he remembers a patient, a 10-year-old boy, who had a rare and devastating neurological disease. The child’s doctors knew he wouldn’t survive. “As the kid got to the point of being ready to die, he was put on the ward. He was never going to get out and go home,” Morens said. “The head of our infectious disease team got permission to infect this kid with malaria to try to get his temperature up to 107, in hopes that it would kill the virus.”

It didn’t work. But the fact that fever as a treatment was even attempted in recent decades highlights the sometimes bizarre manifestations of fever’s cultural standing. “He knew almost certainly it wouldn’t have worked but he wanted to try to do it, even [as recently as] 1972,” Morens said. “I’m not aware that any disease can be killed by a fever. Although it’s said that one of the reasons we have fevers at all is that it potentially kills microorganisms—I think in general that’s not true.”

What, then, is the evolutionary purpose of a fever?

It may simply be a way for humans to know that they’re sick—which is useful not just for the person suffering, but for the community around that person. Consider, for example, what happened during the catastrophic measles epidemic in Fiji in 1875. “These people had never seen an epidemic fever—not ever,” Morens said. “This is a group of people who lived for over 1,000 years on a group of islands. There had never been an epidemic febrile disease there.” And yet they knew how to protect themselves right away.

“First of all, they were terrified—all of the sudden their bodies were turning hot and cold,” Morens said. “They didn’t know what it was or why. They thought they’d been bewitched by spirits... But they immediately undersotod the concept of contagion and they started isolating themselves. They’d never seen a fever before but they immediately recognized it was contagious. Groups of people ran to the highlands and hid until the epidemic went away. In some cases, where people were sick, they locked them up in a village and burned them down and killed them.

“There are many example of that,” Morens told me. “We think of contagion as a sophisticated concept but, in fact, primitive people who have never seen a contagious disease before immediately understand it. That’s been shown again and again and again in history.”

Over time, Western medicine has promoted more pragmatic understanding of fever. Doctors now know that, with the exception of ultra-high fevers of 105 or more, a higher temperature does not necessarily mean a person is sicker. Most cultures have abandoned theologic interpretations—like the idea that a fever was a sign of the gods striking you down, as ancient Egyptians believed. But a deep-rooted fear of fevers remains, passed down from generations of people who lived with a different understanding of their bodies and their vulnerabilities.

“The idea that fever was a big deal has been around for thousands of years,” Morens said. “Fever was a real disease to people, and we carry that legacy forward.”