Forget everything you know about “normal” body temperature and fever, starting with 98.6. There’s no single number for normal. It’s slightly higher for women than men. It’s higher for children than adults. And it is lowest in the morning. 98.6 is an antiquated number based on a flawed study from 1868. That number was the work of Carl Reinhold August Wunderlich, a 19th-century German physician who wrote a seminal text using data from 25,000 patients. He concluded that 98.6 degrees is the body’s normal “physiologic point,” and that fever begins at 100.4 degrees. Wunderlich took patients’ temperatures under the arm, a method that produces readings that are lower (and less reliable) than temperatures taken orally, offsetting some of the disparity.
The facts about fever are a lot more complicated. A new study, published online last month in the Journal of General Internal Medicine, refutes the age-old benchmark of 98.6 degrees Fahrenheit. Instead, the study found an average normal temperature in adults of 97.7 degrees, as measured with an oral thermometer. As for fever, the study shows that it begins at 99.5 degrees, on average.
“A temperature of 99 at six o’clock in the morning is very abnormal, whereas that same temperature at four o’clock in the afternoon can be totally normal,” says Jonathan Hausmann, a rheumatologist at Boston Children’s Hospital and Beth Israel Deaconess Medical Center in Boston, who gathered 11,458 temperatures in crowdsourced research using an iPhone app called Feverprints.
Hausmann believes body temperature to be a flexible concept, viewed in context with age, gender, time of day, and other factors—much in the way weight is evaluated based on height, and how the thresholds for normal blood pressure differ based on age.
In the body’s first response to pathogens, proteins called pyrogens flow through the bloodstream to the hypothalamus, which responds by ramping up the heat. Fever helps your body fight infection by stimulating the immune system, sending a kind of alert to the body’s defenses. It also creates a more hostile environment for bacteria and viruses, making it more difficult for them to replicate.