Running Hot The Story of Fever and the Myth of 98.6

Imagine you are a physician in Leipzig, Germany, in the year 1851. You have just received a new instrument: a glass tube filled with mercury, roughly the length of a ruler. Your colleagues are skeptical. They have been reading pulses, studying urine, and pressing their palms against fevered brows for centuries. They already know, or believe they know, what a sick body feels like. But you suspect that what you feel with your hand and what is actually happening inside the body are two very different things. So you begin to measure. Patient by patient, ward by ward, you place this unwieldy instrument in the armpit of the sick and the well, and you wait. You record the number. You wait again. You record again. And slowly, over years, something extraordinary emerges from the data: a map. Not of rivers and mountains, but of the interior of the human body itself.

This is the story of how humanity learned to read the body's oldest language. It is a story about numbers that turned out to be more complicated than anyone expected, about fevers that turned out to be far wiser than anyone imagined, and about what the rising and falling of body heat can tell us about the boundary between life and death. It begins with a Florentine genius playing with water and air, runs through the wards of nineteenth-century Germany, winds into evolutionary biology, and arrives, finally, at the bedsides of modern intensive care units where doctors are still wrestling with the same ancient questions in urgent new forms.

The temperature of the human body, it turns out, is not a fixed fact. It is a story. And like all the best stories, it keeps changing every time we think we understand it.