The authors are members of the Harvard Working Group on New and Resurgent Diseases, scientists and practitioners who have been working together since 1991. The members are: ecologist Richard Levins, biomathematician Tamara Awerbuch, international health epidemiologist Uwe Brinkmann, philosopher of science lrina Eckardt, medical practitioner Paul Epstein, socialogist of science Najwa Makhoul, social scientist Cristina A. de Possas, systems ecologist Charles Puccia, public-health entomologist Andrew Spielman and clinical infectious-disease specialist Mary E. Wilson. They have recently compiled Disease in Evolution, a book of conference proceedings to be published in 1994 by the New York Academy of Sciences. After this article was completed, Uwe Brinkmann died suddenly in June 1993 while on a site visit in Brazil. Address for Levins: Department of Population and International Health, Harvard School of Public Health, Boston, MA 02115.