If viruses were merely the foreign toxic agents that their name suggests (the word “virus” is derived from a Latin word that connotes poison and sliminess) then most aspects of their natural history could be described by relatively straightforward dose-dependent curves and we would relegate the study of their properties to the disciplines of toxicology and molecular pharmacology. Viruses can, of course, destroy cells and tissues: many examples can be cited in evidence of how viruses become intricately antagonistic to the cell’s defenses. 28 , 63 Thus, as a recent popular book describes them, they are “invaders—[which] cause more sickness than anything else on earth.” 69 But, interestingly enough, the same book also describes them as “co-travelers in life,” which suggests a fundamental ambivalence about their nature. Indeed, viruses can be viewed as adaptations of self, thus as Lewis Thomas suggested some years ago “viruses, instead of being single-minded agents of disease and death, now begin to look more like mobile genes.” 89 Thomas’s insight is astonishing given that it was reached without our contemporary knowledge that a relatively large fraction of the human genome is viral in origin. 44 Furthermore, we now know about the inclusion of human genes in viral genomic molecules (oncogenes being the most 2studied cases). 3 , 16 For us, the classic question of whether viruses are living or not has been supplanted by the question of whether viruses are self or (like bacteria) clearly other.