98.6 degrees. That's what we grow up being told the temperature of the human body is—if you use Fahrenheit, anyway; that same temperature is 37 in Celsius. That’s a weirdly specific value. So specific, we should have been skeptical of it. The body is pretty good at regulating itself and keeping conditions constant, but can it really keep you at an exact temperature, correct to one-tenth of a degree?

No, it can't. 98.6 degrees actually represents the average temperature of a whole bunch of people. One scientist named Carl Wunderlich came up with that number in 1851 by taking people's temperatures repeatedly. He studied 25,000 people many times each, taking around a million measurements, and 98.6 was the mean of his many readings. People's temperatures vary from the mean in both directions, as you might have noticed if guards at checkpoints have taken your temperature these last couple years for some reason. Your temperature also fluctuates quite a bit even over the course of each day.

Side note: Some sources say Wunderlich proclaimed a mean temperature of 37 degrees Celsius, meaning roughly 37, or "somewhere between 36.5 and 37.4 but rounded to the nearest degree." If that were true, it would be silly to convert it to Fahrenheit with so much precision that you get 98.6 degrees. It would be like converting "I would walk 500 miles" to "I would walk 804.7 kilometers." However, Wunderlich did not round his result like that. He came up with the value 37.0 degrees Celsius—his readings just happened to average out to a whole number. So 98.6 isn't wrong for that reason. It's wrong for other reasons. 

Though Wunderlich had a lot of data, it wasn't very good data. He took temperatures of the armpit, which isn't the best spot compared to the mouth or anywhere more internal. And his thermometer, recent analysis suggests, wasn't even calibrated correctly

Other scientists have tried calculating the mean human body temperature since Wunderlich did. Invariably, they come up with an answer lower than 98.6. The mean human body temperature is below 98 degrees. In fact, it seems that every time scientists calculate a new mean, they come up with a slightly lower value than a few decades before. That's because when you examine thousands of people, a few of them are bound to be a little sick and have higher than normal temperatures. The healthier that people in general get, the fewer of these feverish people slip into the sample, and the lower the mean temperature falls. 

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