Who Lives in a Drop of Water?

If you can't see it, it isn't there

For a very long time that is what people thought. The smallest living thing was something like a flea. Anything smaller was just dust.

In the 1660s a man in London stacked several lenses and looked at a flea. The flea wore armour: joints, hairs, hooked claws. He drew it large and folded the drawing into a book. Unfold it and the flea is as long as your arm. People saw, for the first time, the face of a thing that had lived on their own bodies.

He also sliced cork thin and looked. It was packed with tiny rooms, like a honeycomb. He borrowed the word for a monk's small room and called them cells. The name stuck.

The cloth merchant's glass bead

But the real surprise was seen by someone else: a cloth merchant in Delft. No university, no Latin. He used a magnifier to count threads, and fell into grinding lenses.

His lens was a single tiny glass bead. Yet it magnified more than two hundred times. He looked at a drop of rainwater. Something was swimming. Not one thing — the drop was full.

Then he scraped between his own teeth and looked. They were there too. He wrote that more of them lived in one man's mouth than there were people in a kingdom. The scholars in London did not believe him. In the end they sent someone to Delft to look for themselves.

How small?

Picture one hair. Line bacteria up across its width and fifty to a hundred fit. And bacteria are not the smallest. There are things one hundredth the size of a bacterium. That story comes in piece 3.

Small does not mean few. Gather every microbe on Earth onto a scale and it outweighs every animal on the planet put together — elephants, whales and people included.

So where on your own body do they live, and how many? Not just the mouth. Tap the body.

Tool The Body Microbe Map — Where Your Tenants Live Tap a spot on the body and see who lives there, how many, what they do and what they like. The gut is the capital, the skin a desert, the armpit an oasis. One common myth corrected at every spot, plus a gallery of microbe faces, a plate game and a myth quiz. (Korean only)

A little further in

Seen, and not understood, for 200 years

The merchant saw the swimming things in the 1670s. It took another two hundred years to learn that these things cause disease and ripen food. Why so long?

Because that something exists and what it does are different questions. Seeing things swim in water does not tell you they spoil milk. Joining the two needed other tools — scales, flasks, and stubborn people. That is where the next pieces go.

So what do these tiny things do? Let's go to the kitchen. On a summer day the kimchi jar fizzes. Open a bottle of makgeolli and it foams over. Who is doing that?

The question that remainsIf we lived with them all along and only saw them 300 years ago — what is beside us now that we still have not seen?