Looks like there a new normal.
Researchers who took hundreds of thousands of temperature readings found that 97.5 was the new normal, down about a degree from what German physician Dr. Carl Wunderlich established in 1867 in a study of 25,000 people. (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% of normal temperatures are now below 98.6.
Anthropologists Michael Gurven and Thomas Kraft, at the University of California, Santa Barbara, who wrote an article, "People's Bodies Now Run Cooler Than 'Normal' — Even in the Bolivian Amazon," noted: "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 as you age.
But if your temperature usually falls lower than average, does 98.6 mean you have a fever? Possibly, said Sharon S. Evans, a professor of oncology and immunology 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, 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 warmblooded and coldblooded," she said. "Fever affects every aspect of the immune system to make it work better."
For starters, 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. "Fever enhances survival." That accounts for its persistence throughout animal evolution, even though it exacts a significant metabolic cost. Immunity "works better at higher temperatures."
A fever reducer would probably make you feel better, but Offit said, "You're not supposed to feel better. You're supposed to stay under the covers," not go out and spread it to others. Fever helps to reduce viral shedding and shorten the length of illnesses like the flu.
Grandma's proverbial cold remedy of hot chicken soup probably helps because the steam raises the temperature of nasal passages, repressing reproduction of the virus, he said.