This is 99% invisible. I'm Roman Mars.

Imagine for a moment the year 1800. A doctor is meeting with a patient,

most likely in the patient's home.

The patient is complaining about shortness of breath, a cough, a fever.

The doctor listens to the patient's complaints, asks a ton of questions, scribbles some notes,

producer Emmett Fitzgerald, and the doctor might check the patient's pulse,

but unlike today what's happening inside of the patient is basically unknowable.

There's no MRI, no x-rays.

The living body is like a black box that can't be opened. The only way for a doctor to figure out

what's wrong with the patient is to ask them. You needed the patient to tell you what

was wrong and what they were suffering. So doctors were very interested in the patient's history,

the story of what the symptom was,

how long it had been there, how had it changed,

what made it better, what made it worse.

This is Dr. Jacqueline Duffin. I am a hematologist and a

historian of medicine at Queen's University. Duffin says that because the

patient's story was all the doctor had to go on,

the symptoms were often seen as diseases in themselves.

So, for example, today if you have a fever,

you think of it as a symptom of some underlying disease like the flu.

But back at the turn of the 19th century, the fever itself was considered the disease.

There were all different kinds of fevers, and fever itself was a broad category of diagnosis.

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