Viruses have been around longer than PCs, and are not without a certain mathematical and scientific interest. Indeed, not all viruses are malignant. Used properly, viral techniques are a valuable programming tool. Used improperly, they are pestilentially destructive. There’s no perfect cure. Like the flu, computer viruses evolve. Last year’s immunization isn’t any good for this year’s disease because every time someone invents a new medication, someone else invents a malady. Nonetheless, a few simple precautions will buffer you against all but the cleverest hacker.