They teem in a sub-microscopic world that has flourished almost since the dawn of time. They’re barely a life form of their own — stripped-down flecks of genetic material encased in protein. These particles are so minuscule that 100 million of them might fit on the head of pin — and that tiny mess itself could be contagious enough to infect and sicken 1,000 of us.

The largest virus is smaller than the tiniest bacteria, so tiny they cannot even be seen with a regular light microscope. (The photo here was snapped with an electron microscope.) And unlike bacteria, viruses, as far as we know, are never good for you.

Welcome to the all-too-close but little-known world of viruses....