FRAGMENT FROM NATURE continues from last weekend on the positive impact of viruses on life on this planet. For a very long time, viruses have only been associated with diseases, and little else was known about them. But few are pathogenic, and the rest have played either benign or astonishingly advantageous roles during evolution. We still have a long way to go in understanding them. Just look at the composition of our own genomes, replete with viral footprints. What does this mean?