To the Editor: Computer viruses are designed to be pests, proliferating in uncontrolled ways and causing severe damage to electronic data. These malignant programs, which amplify between files and computers, are strikingly similar in virulence, modes of spread, and evolutionary pathways over time to the microbes that cause infectious diseases. Both biological viruses and these virtual viruses are transmitted from host to host. Computer viruses are a human invention; however, their development follows a well-recognized biological route. Relatively harmless ancestors gradually or step-by-step evolve into “pathogens;” the host develops adaptive defense mechanisms, which in turn select for new virus “variants;” eventually, equilibrium is reached between infection and host defenses. Comparing “virtual microbes” with their biological counterparts can help us control both.