The following is an encore presentation of Everything Everywhere Daily.

Over thousands of years, many theories have been put forward as to the cause of communicable diseases.

These theories range from the religious to the magical and sometimes quasi-scientific.

But what they all had in common was that there was no proof for anything.

Over the centuries, these theories became dogma and often prevented a better understanding of

diseases. It wasn't until the late 19th century that we got a clear picture of what the cause

actually was. Learn more about the germ theory of disease and why it took so long to recognize on this episode of Everything Everywhere Daily.

I'm David Ramnik, host of The New Yorker Radio Hour.

There's nothing like finding a story you can really sink into that lets you tune out the noise and focus on what matters.

In print or here on the podcast, The New Yorker brings you thoughtfulness and depth and even humor that you can't find anywhere else. So please join me every week for the New Yorker Radio Hour,

wherever you listen to podcasts.

Before I begin, I should clarify a few things about the phrase germ theory of disease

because each of those words can be a bit confusing.

For starters, germ is just a catch-all term for anything microscopic that can cause disease,

including bacteria and viruses.

There is no set scientific definition of germ,

and the term usually isn't used in research circles. Next is the word theory. A theory does not

mean that something is unproven. It is simply a set of statements or principles devised to

explain a group of facts. The theory of gravity is still called a theory, even though

we have a pretty good understanding of how gravity works. And the final word that needs to be

clarified is disease. For the purpose of this episode, disease will refer specifically to

communicable diseases. These are diseases that are spread from one person to another.

...