Long ago -- around the time we started growing our own food - humans settled down.

We went home, inside.

We built permanent shelters to protect us from the elements... and keep the wild animals at bay.

Or so we thought.

Surprise!

The animals were right there with us.

They still are.

This is dust.

Zoom in and you find an ecosystem almost as elaborate as the one we left outside.

But small enough for us to forget it exists.

Dust is pretty much anything small.

But the most important ingredient of dust - - at least for the purposes of this story - - is skin.

Your skin.

Her skin.

His skin.

Tiny flakes that fall off our bodies.. all day long.

Researchers at The California Academy of Sciences in San Francisco collect and study house dust to find out what, exactly makes up this micro-universe.

Even the cleanest homes are teeming with tiny, almost invisible roommates.

- - and even more so if you have pets or kids or live on the ground floor.

Most homes have over 100 species - no matter how often you vacuum.

Not just these guys, but these and these.

Most of these microscopic roommates are harmless.

Just freeloaders, basically.

But one can cause real trouble: the house dust mite.

This is like the roommate who leaves his crap around and makes you sick.

Dust mites don't bite people.

They don't need to.

We feed them.

Constantly.

Skin flakes are hard to digest.

It's like eating hair or feathers.

So dust mites have powerful digestive enzymes to break the skin down.

Those enzymes turn up in dust mite poop.

And let's just say you probably don't want to know how much dust mite poop is in your house..

When people breathe dust, they breathe in the poop -- and the enzymes, too -- which irritate the lungs and can aggravate asthma, especially in kids.

Like us humans, dust mites haven't always lived inside either.

These tiny relatives of spiders and scorpions once lived in birds nests.

But then, some intrepid dust mites made the jump... from bird's homes, to ours.. And as our society thrived and grew, so did theirs.

Hi - this is Amy.

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