Introduction

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.