A reasonably informative overview dealing with the discovery of ""nonfilterable viruses""; their makeup; how they invade cells, and how cells fight back; the different viruses that infect bacteria, plants, animals, and humans, and the different families of each; and what we know of viruses' role in cancer and other diseases. In process of discussing viruses at work, Knight must explain the structure and components of nucleic acid, and this could be clearer. So, at times, could Knight's sentence structure and his parenthetical disposal of unfamiliar terms. However, he does well enough with this difficult material to carry later discussions of virus and cell behavior. His survey of current approaches to fighting viral disease is enlivened by reference to several suggestive studies, and the diagrams and electron-microscope photos add dimension.