Under most circumstances, fever is beneficial, reducing the severity of illness and shortening its length.
Among the many measures my local Y is using to prevent the spread of Covid-19, instant temperatures are taken with a forehead scanner before people can enter the building. Curious to know how “hot” I was one cold, rainy day, I asked the attendant what it registered: 96.2.
The last time my temperature was checked in a medical setting it was 97.5. Whatever happened to 98.6, the degrees Fahrenheit that I and most doctors have long considered normal body temperature?
As if reading my mind, Dr. Philippa Gordon, a Brooklyn pediatrician, sent me an article, “People’s Bodies Now Run Cooler Than ‘Normal’ — Even in the Bolivian Amazon,” by two anthropologists, Michael Gurven and Thomas Kraft, at the University of California, Santa Barbara.
As they wrote in The Conversation, “There is no single universal ‘normal’ body temperature for everyone at all times.” Rather, body temperature varies, not only from one person to another, but also over the course of the day — lower in the morning, higher in the evening; rising during and after exercise; varying at different times in the menstrual cycle, and at different ages — lower for old-timers like me. Aha!
Furthermore, researchers who took hundreds of thousands of temperature readings from people in Palo Alto, Calif., found that 97.5 was the new normal, down about a degree from what the German physician Dr. Carl Wunderlich established in 1867 in a study of 25,000 people. (Dr. Wunderlich’s research did find that “normal” body temperature ranged from 97.2 to 99.5.)
In reviewing data from 1862 through 2017, Dr. Julie Parsonnet, a professor of medicine at Stanford University School of Medicine, and co-authors found a steady decline in average body temperature of about 0.05 degrees Fahrenheit per decade. She has observed that at least 75 percent of normal temperatures are now below 98.6.
If my body temperature registered 98.6, would that mean I have a fever? Possibly, said Sharon S. Evans, a professor of oncology and immunology at Roswell Park Comprehensive Cancer Center in Buffalo, N.Y., even though 100.4 is generally considered the lower end of the fever spectrum.
In a review written with two colleagues, Elizabeth A. Repasky and Daniel T. Fisher, Dr. Evans showed that under most circumstances, fever is beneficial, reducing the severity of illness and shortening its length. (She emphasized, however, that patients should follow their doctors’ advice about taking medications to reduce fever.)
“Fever acts to mobilize multiple arms on the immune system, a function that is remarkably well conserved across many, many species — both warm-blooded and coldblooded,” she explained in an interview. “Fever affects every aspect of the immune system to make it work better.”
For starters, Dr. Evans said, fever activates innate immunity — the mobilization of white blood cells: neutrophils that patrol the body for pathogens and macrophages that gobble them up. Macrophages, in turn, send out an alarm that help is needed, prompting adaptive immunity — T cells and B cells — into action. These cells initiate a specific response to the invader: the production of antibodies days later.
“Treating fever can prolong or worsen illness,” Dr. Paul Offit, vaccinologist at the University of Pennsylvania, stated in “Hippocrates Was Right: Treating Fever Is a Bad Idea,” a fascinating YouTube presentation by the College of Physicians of Philadelphia.
“Fever enhances survival,” Dr. Offit reported. That accounts for its persistence throughout animal evolution, even though it exacts a significant metabolic cost. Immunity, both innate and adaptive, “works better at higher temperatures,” he said.
Thus, when you take medication like acetaminophen (Tylenol and its generic forms) or ibuprofen to suppress a fever, you actually work against the inherent protective benefits nature bestowed. Yes, a fever reducer would likely make you feel better, relieving symptoms like headache, muscle aches and fatigue. But, Dr. Offit emphasized, “You’re not supposed to feel better. You’re supposed to stay under the covers, keep warm and ride out the infection,” not go out and spread it to others.
“We have fevers for a reason,” he said. Fever helps to reduce viral shedding and shorten the length of illnesses like the flu.
Grandma’s proverbial common cold remedy of hot chicken soup likely helps because the steam raises the temperature of nasal passages, repressing reproduction of the virus, he suggested.
Dr. Evans and colleagues wrote, “The fact that fever has been retained throughout vertebrate evolution strongly argues that febrile temperatures confer a survival advantage.” This is true for invertebrates like insects as well. And when coldblooded animals like lizards or bees get sick, they try to raise their body temperature by increasing physical activity or seeking a warmer environment, Dr. Evans said.
So why are we so hellbent on suppressing fevers? Fear is one reason, said Dr. Gordon, the Brooklyn pediatrician, who said frantic parents often call in the middle of the night when a child’s fever spikes. She suggested that doctors warn parents ahead of time to expect a nighttime rise in a child’s fever and explain that high fevers from an infection are not damaging.
“The body has a built-in thermostat — the hypothalamus — that keeps temperatures from getting high enough to cause damage,” she said, and febrile seizures (brief convulsions, shaking and perhaps loss of consciousness that affect some young children) result from how fast temperatures rise, not how high they get. In a genetically susceptible child, a seizure can occur when the temperature rises quickly even at low temperatures, say, from 99 to 100.8.
“Febrile seizures are creepy and terrifying for parents to watch, but they don’t cause any damage,” Dr. Gordon said. She added, however, that fever is worrisome in very young babies who have immature immune systems and haven’t yet been vaccinated against serious diseases.
Even knowing the infection-fighting benefits of fever, Dr. Gordon said she would still recommend medication to lower a fever if a child, especially a nonverbal child, is very miserable and perhaps unable to sleep or eat.
Adults are generally advised to seek medical help if their fever rises above 103 degrees.
An important caveat about high fevers: Unlike fevers resulting from an infection, there is no natural shut-off for environmentally induced fevers, such as might occur if a child is shut in a hot car or an overdressed athlete overexercises on a hot day, which can result in fatal heat stroke.
Consumers should also consider how a temperature is taken before interpreting the results. An ear temperature is usually slightly higher than an oral temperature, which in turn is higher than an armpit temperature or a forehead scan.
To assure a reliable reading, the temperature of newborns should be taken with a rectal thermometer, Dr. Gordon said. But for older children with a fever, the exact number of degrees doesn’t really matter unless they’ve not been immunized, she said.
