For more information, send e-mail to permissions@press.princeton.edu This file is also available in Adobe Acrobat PDF format INTRODUCTION Felicia Keesing, Richard S. Ostfeld, and Valerie T. Eviner PATHOGENS ARE UBIQUITOUS. We are all familiar with the cold viruses that give us sniffles, coughs, and aches, and with the more frightening pathogens that cause diseases such as AIDS, malaria, and tuberc