Fevers aren’t always a sign of a passing virus or some other benign illness. Among the the more perplexing conditions for doctors are persistent fevers that seem to have no cause, or fevers of unknown origin. Startlingly, many physicians still don't have a clear understanding of how to manage fevers, despite them being among the most common health problems. A 2001 study found a significant number of doctors demonstrated a “serious lack of knowledge” of the nature, dangers, and management of fever in children.

Avicenna, it should be pointed out, did not have it right about fevers, either. Physiologically, they are not “kindled in the heart,” but triggered by a reaction in the hypothalamus, a bundle of tiny nuclei about the weight of a penny that sits just above the brainstem. “Going back to Hippocrates, people—doctors and their patients—had the conception that fever was the disease itself rather than, say, caused by salmonella, or influenza, or some microbial organism,” Markel told me. “Nobody knew about microbes, so fever was considered a disease.”

In those days, most people weren’t as concerned with where a fever came from as they were with what might happen to you if you got one. “Very often you tended to have infectious diseases that killed you,” Markel said. “That’s one reason why fever would be terrifying.”

“If you have a high fever and it’s before the era of antibiotics, you’ll be sick as hell,” he continued. “With pneumonia, which was extremely common, you could have a really high fever of 104 or 105. People died of it. That’s scary stuff. And your body either fights it off or it doesn’t. Sometimes, with diseases like bacterial pneumonia, you’d be sweating, your fever would be high, and you’d either resolve it or you’d die. These were very dramatic moments.”



Evidence of these moments was everywhere, culturally. In the 18th and 19th centuries, newspaper pages were cluttered with ads for tonics promising to “cure fever.” And because “fever” was a euphemism for a number of serious illnesses, stories of people who “died from fever” were all over obituary pages.

“Back then, people said, ‘Fever came to town,’ and ‘This year’s fever had a lot of red rash.’ We now know that meant measles,” said David Morens, a senior scientific advisor at the National Institute of Allergy and Infectious Diseases. “When they said, ‘Last year fever came to town and it had a lot of vesicular rash,’ we now know that was smallpox. They didn’t understand fever in its different forms as indicating any distinct disease that was different from any other disease.”

Which is why generations of doctors tried to treat fevers themselves, not their underlying causes, often with gruesome outcomes. “Particularly with febrile diseases, one of the ideas was that you had unbalanced humors,” Morens said. “The idea was that bad humors were causing the disease, or at least the fever, and you had to get rid of the poison.” In the 1700s, patients with febrile diseases were bled, sometimes to death, in an attempt to get rid of toxicity. Other treatments included medicines that made a person vomit, sweat, or have diarrhea.