Follow this link and click on the animation (under Related Projects) to see a bacteriophage landing on and injecting DNA into an E. coli bacterium. If you have a slow connection, it may take a few minutes to load.
You are looking at the most common act of aggression on Earth.
A bacteriophage (phagein, "to eat"), or simply phage, is a virus that feeds on bacteria. A typical phage, such as the one in the animation, looks a bit like a lunar lander, or some sort of extraterrestrial bot. Ten thousand of them could lie down end to end across the period at the end of this sentence. Like other viruses, they can hardly be called alive, since they are inert except when in interaction with a host.
Phages waft aimlessly about, and now and then manage to bind with their spidery "legs" to a host cell that has the proper surface receptors. Then, as in the animation, it squats and injects a snippet of viral DNA into the cell. The viral DNA replicates and builds more viruses, until the host cell is filled to bursting.
According to an article by Ryland F. Young III in a recent issue of Science (August 15), there are upwards of 1030 phage infection a day in Earth's biosphere -- a number so colossally large as to defy imagination. Quantitatively, phage aggression is far and away the dominant predator-prey relationship on the planet. "Within 2 hours of the addition of a single T7 bacteriophage particle to a culture of 10 billion Escherichia coli cells, more than 99.9% of the bacteria are destroyed and 10 trillion virus particles are generated," says Young. Take another look at the animation. Imagine a biosphere teeming with those myriads of spindly-legged robots -- land, squat, squirt -- land, squat, squirt -- 10 billion in a teaspoon of sea water, not quite alive but not quiet dead either, the stripped-down primal machinery of life, hell bent on making more of itself.
Of course, bacteria never cease to evolve defenses, and phages counter by finding their own useful mutations. As I write, I am battling my own viral infection, the common cold. The table here by the couch has its array of tissues, NyQuil, and cough drops, a pathetic armory of prophylactics that won't do a bit of good. I must leave it up to my body's natural defenses, evolved over the ages -- my own local version of the epic warfare of phages and bacteria that is the ubiquitous backdrop of life on Earth.
I don't have to draw the moral of this story. Land, squat, squirt. It's been going on for three billion years, and it'll be going on long after we are gone.