"New diseases" seem to keep arising out of nowhere to threaten humanity. But actually, as the authors show us, the emergence of diseases (parasite-borne, plagues) is simply one of the logical outcomes of life itself. While their total eradication may be impossible, we can make sure we are well prepared for a struggle unlikely to disappear. Professor Patrice Debré is head of immunology at Pitié-Salpêtrière Hospital in Paris. Virologist Jean-Paul Gonzalez is Director general of the International Medical Research Centre of Franceville, Gabon.