The vermin only teaze and pinchTheir foes superior by an inch. So, naturalists observe, a flea Has smaller fleas that on him prey; And these have smaller still to bite 'em, And so proceed ad infinitum*: -Jonathan Swift
Even though I study the immune system, it always amazes me just how many creatures make their living by parasitizing other creatures. It's like the food chain turned on its head. The food chain you probably learned about in school says that many organisms feed on other organisms, which get fed on by others until you reach whatever ultimate predator sits at the top of the food chain.
But even the lion can get infested with parasitic worms, or bacterial infections. And those worms and bacteria can get infected with viruses, and they do. Carl Zimmer has a typically great post about the viruses that prey on the bacteria that live in our mouths, and how scientists are beginning to unravel the constant battle that exists in creatures too tiny to be seen.
These microbes live in our guts, lungs, mouths, noses, skin, and many other nooks and crannies. Far from making us sick, they help us in many ways, making food for us, defending us from invaders, and nurturing our immune systems.
These bacteria are also hosts to viruses. In World War I, the Canadian doctor Felix d'Herelle discovered the first virus infecting bacteria while studying the stool of French soldiers sick with dysentery. Once he isolated the bacteria-attacking virus, he could use it to destroy cultures of the dysentery-causing bacteria.
He goes on to describe how scientists are using special DNA elements found in bacteria that have fended off viral attack as a sort of historical record, documenting battles faught over time in the mouths of several individuals. Check it out!
*These lines are so burned into the psyche of microbiologists that we almost named this blog "Ad Infinitum."