This flu season, as always, many people will be reaching for the thermometer to see whether their temperature has surpassed that familiar, call-out-sick, stay-home-from-school benchmark: 98.6 degrees.

That number became the norm thanks to the work of a single German doctor working in the 1800s -- even though more recent studies show that it’s probably too high.

In 1851, Carl Reinhold August Wunderlich decided to start using a new instrument in his clinic: the thermometer. He was among very few using the device, although he wasn’t the first: Thirty years earlier, a pair of French doctors had shown that inflamed body parts and exercising muscles were hotter than the rest of the body. (They showed the latter by sticking a heat-sensitive needle in a man’s arm while he sawed wood.)

They had also suggested that the average human body temperature was about 98.5 degrees.


Wunderlich set out to prove it. Using a foot-long thermometer that took more than 15 minutes to give a reading, he took the underarm temperature of 25,000 patients several times over -- a total of more than a million readings.

(That’s more than 250,000 hours, or 10,000 days, of temperature-taking, for those who are counting.)

In an astounding feat of patience (given that calculators were unimaginable at the time), Wunderlich then averaged the temperatures and came up with 98.6 degrees.

The number stuck -- although it’s starting to give way.


Since Wunderlich’s day, doctors and researchers have observed that normal body temperature varies from person to person and changes depending on the time of day. In 1992, three doctors working at a Veterans Affairs hospital in Baltimore took several daily temperature measurements of 148 men and women over three days.

In a now oft-cited paper, they published their results: The average temperature was 98.2.

But the doctors, led by Dr. Philip Mackowiak, now a professor of medicine at the University of Maryland School of Medicine, were hesitant to put too much stock in this average. Like Wunderlich more than a century before, they found that body temperature rose and fell during the day, from a low at 6 a.m. to a peak at around 6 p.m.

Mackowiak’s team found other influences on temperature variation. Women in the study were slightly hotter than men (98.4 versus 98.1), and blacks were slightly warmer than whites (98.2 versus 98.1).


Other researchers have shown that age also matters: Children are warmer than adults, thanks to their rapid metabolism, and that cooling trend appears to continue throughout life. A 2005 study showed that in 150 seniors, the average temperature was 97.3 degrees early in the morning and 97.8 just before bed.

All this has left many puzzled as to how Wunderlich came up with the number 98.6. One answer seems plausible. After Mackowiak announced his results, he was contacted by a museum of medical artifacts in Philadelphia, which had a centigrade thermometer thought to have been used by the German doctor for his experiments.

Tinkering with the unwieldy instrument, Mackowiak noticed something: The antiquated thermometer used to calculate a norm that the field of medicine has relied on for more than a century was calibrated 1.5 degrees Celsius (roughly 3 degrees Fahrenheit) too high.