As public health officials struggle to contain the spread of the coronavirus, determining whether a person has a fever is now a high-stakes matter, and using temperature guns to screen people has become a visible strategy for detecting possible cases. “Any infectious disease — one of the cardinal signs of infection is raised body temperature,” says Waleed Javaid, the director of infection prevention and control for the Mount Sinai Downtown Network. But there’s a catch: “That means you know the body temperature before you raised it.”

Body temperature varies considerably both among and within people, based on weight, height, physical activity, the weather, clothing. Older people tend to be cooler than younger people. A recent review suggests women might tend to be slightly warmer than men. Temperature also fluctuates with the sleep-wake cycle. Of course, public health officials can’t know each person’s usual thermal patterns, so their advice is based on population averages. Yet those numbers aren’t consistent, either, and most refer to oral temperature, which is considered more accurate than an infrared scan of the forehead. The Centers for Disease Control and Prevention considers above 100 to be a flu-related fever. The National Institutes of Health says an adult with a temperature above 99 “probably” has a fever, depending on the time of day. Before the current pandemic, hospitals would generally assess fever and act upon it differently in response to individual patients and diagnoses; schools have had various thresholds to determine when parents should keep their children home. Perhaps the most well-known benchmark is 98.6 degrees, which many people, including physicians and public health agencies, consider to be a baseline. “All of us think back to our mothers taking our temperature and saying: ‘It’s 98.6. You’re going to school,’” says Dr. Julie Parsonnet, an infectious-diseases specialist at Stanford University.

That number dates to 1851, when a German physician named Carl Reinhold August Wunderlich began taking what he purported were millions of temperature readings from 25,000 patients in Leipzig. Their average, he announced, was the Celsius equivalent of 98.6. That figure, which he defined as “normal,” has persisted, despite numerous more recent studies that put the average closer to 97.88 or lower. A 1992 study in JAMA suggested that the discrepancy could be explained by Wunderlich’s primitive thermometer or other external variables. But Parsonnet wondered if our average body temperature might have decreased. Initially, she couldn’t find records going back far enough to find out. Then in 2018, the National Archive of Computerized Data on Aging received an update: medical records of veterans of the Union Army taken from 1862 until 1940 that contained 83,900 temperature readings. Parsonnet and colleagues analyzed the data according to the veterans’ birth decades and found that average body temperature steadily decreased by about .02 degree Celsius per decade; veterans born more recently were cooler. These data were compared with data collected between 1971 and ’75 and between 2007 and ’17. Over all, a similar pattern emerged: The average temperature of men born between 1800 and 1997 decreased by about .03 degree per birth decade. (Women born between 1890 and 1997 had a .029 per decade decrease but weren’t represented in the Union Army data.)

The most likely reason for such a change, Parsonnet and co-authors argue in a study published in January in eLife, a journal of biology and medicine, is that improvements in living standards, sanitation and medical care have reduced the number of cases of untreated chronic illness, including tuberculosis, syphilis and periodontitis. Fighting infection increases a person’s basal metabolic rate — the amount of energy needed at rest per unit of time — and that raises his or her temperature. A lower average temperature therefore suggests a lower average metabolic rate. And a lower metabolic rate tends to correlate with a greater body mass, at least in animals. So far, researchers can only speculate about possible connections between a lower average temperature and rising obesity rates in people. (Many variables other than infection could have influenced the results: To take one example, a growing number of people live in homes with heating and cooling systems and thus expend less energy than previous generations did to maintain a constant body temperature.)

Such a substantial change in average temperature over a fairly short period of history could have other, unforeseeable impacts. Parsonnet points out that there are “more microbial organisms in us than there are human cells,” which creates a complex ecosystem. And like a human-size version of climate change, “we’re seeing probably a change in our ecosystem that’s associated with this drop in temperature.” Yet we’re only beginning to understand all the ways temperature influences that ecosystem to help determine how we function.

Our body temperature is controlled by the hypothalamus, which acts as a thermostat, keeping the temperature of vital organs fairly constant. (It’s this core temperature that a thermometer approximates.) Temperature sensors in nerve endings, which produce the sensation of being hot or cold, prompt the hypothalamus to initiate adjustments like shivering to warm up or sweating to cool down. At any given time, your skin might be 10 degrees cooler or warmer than your core. And that difference — and thus how much energy the body has to expend to keep the core stable — seems to affect how the immune system functions. For instance, in 2013 Elizabeth Repasky of the Roswell Park Comprehensive Cancer Center and co-authors reported in P.N.A.S. that raising the room temperature improved the ability of laboratory mice to fight off cancer after they got it. Repasky and others are also experimenting with heating tumor cells to kill them or make them more susceptible to chemotherapy. Already, certain abdominal cancers are treated with “hot chemotherapy,” in which the drug is heated to 103 degrees, which has been shown to increase how much of it is absorbed by cancer cells. Separately, the heat from a fever may help fight infection, because, as Mark Dewhirst, an emeritus professor of radiation oncology at the Duke University School of Medicine, puts it, “a lot of bacteria and other pathogens don’t fare well at elevated temperatures.”

Scientists struggle, though, to explain how a cooler average body temperature has been associated with longevity. A lower metabolic rate, and thus a lower temperature, has been linked to a longer life span in experimental settings with reduced calorie intake, when the body slows to conserve energy. But Bruno Conti, a professor of molecular medicine at the Scripps Research Institute, and colleagues have also found that mice genetically engineered to have a body temperature a half-degree lower than average lived longer than ordinary mice, even if they ate as much as they wanted. What other effects this has on an organism is unknown. “For instance,” he says, “a brain at a lower temperature might not function as well.”

At the same time, other bodily systems might benefit from being cooler. H. Craig Heller, a biology professor at Stanford, and colleagues have shown that muscle fatigue is caused by heat, which they believe triggers a temperature-sensitive enzyme that acts as a safety valve, stopping the production of chemicals that power muscle contractions in order to prevent the tissue from burning up. When Heller cools muscle during physical activity using special gloves that chill blood as it moves through the hands, the muscle “just keeps on going,” he says. “I’ve had freshmen doing more than 800 push-ups.”

It’s unclear if or how the coronavirus might change the use of body temperature as a diagnostic tool. In the near term, Javaid says, knowing your own average temperature and how it fluctuates might help clinicians diagnose and treat some illnesses more accurately. He suggests taking it at set times for several weeks and giving that information to your doctor, if needed. “I think what we’ll end up concluding is, just like people know their blood pressure, they also should know what is their temperature when they are normal,” he says. But, Parsonnet adds, as a rule, it’s also important to keep in mind that number’s limitations: “If it’s normal and you feel sick, you’re still sick.”