The virophage as a unique parasite of the giant mimivirus : Abstract : Nature

"So nat'ralists observe, a flea
Hath smaller fleas that on him prey,
And these have smaller fleas that bite 'em,
And so proceed ad infinitum."

Jonathan Swift

To the best of our knowledge the predator-prey sequence doesn't proceed ad infinitum. The smallest "flea" we know of is the virus. Most viruses can only be seen with electron microscopes, but as anyone who has a cold knows, the can pick on some very big hosts. Now researchers have found that viruses themselves can be prey - to other, smaller viruses.

The virophage as a unique parasite of the giant mimivirus