For most of human history, people were infected with intestinal worms as a matter of course. It's only in the past century, in wealthy countries, that mass deworming campaigns and improved sanitation removed them from virtually the entire population. Now some researchers think that eliminating these ancient passengers may have contributed to a sharp rise in autoimmune and allergic diseases.
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The surface of the Earth is the shore of the cosmic ocean. On this shore, we have learned most of what we know. Recently, we have waded a little out to sea, enough to dampen our toes or, at most, wet our ankles. The water seems inviting. The ocean calls.
Carl Sagan — Cosmos
Basically, scientists can't see the proteins inside of a cell in real-time in their natural environment. They can image ~10% of the proteins once they're pulled out and purified, using cryo-electron microscopy (cryo-EM), or < 1% when they're actually working inside a cell, but most of our cell's proteins operate in the shadows.
Packy McCormick
The naturalist hardly expects to be believed when he relates the instincts of animals; yet the physiologist is compelled to believe them, because he cannot explain them away.
William Whewell — The Philosophy of the Inductive Sciences