"So nat'ralists observe, a fleaTo 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.
Hath smaller fleas that on him prey,
And these have smaller fleas that bite 'em,
And so proceed ad infinitum."
The virophage as a unique parasite of the giant mimivirus