Viruses are pure purpose, things on the very edge of life. We can't taste them, smell them or see them, but they're there: in the air, in the water, in every living thing on the planet. They're the tiniest, simplest forms of life on earth, yet they know ways to enter our bodies, kidnap our cells and outwit our immune defenses. Viruses range from the common cold to HIV to H1N1 and have caused epidemics more decimating than wars. Viruses are old. Some believe they're older than all other forms of life. And they are patient. Without a living cell to invade, they wait in a crystalline form, somewhere between life and death. Some can wait for centuries. Their only goal is to reproduce.