Zoonoses, infectious diseases that are transmissible from animals to people, are among the most important animal and public health problems affecting the well-being of societies worldwide. Recent analyses estimate that about 60% of all infectious disease agents affecting humans are zoonotic in origin and that 75% of emerging infectious diseases of humans are zoonotic. Free-ranging wildlife have been an important source of zoonotic infections throughout history and are implicated in most emerging zoonoses. A possible outbreak of bubonic plague transmitted by wild rodents and caused by the bacterium Yersinia pestis was described in the Old Testament in the First Book of Samuel. Similarly, rabies transmitted by a suite of wild animals can be traced back to Chinese, Egyptian, Greek, and Roman records. Wildlife were involved in the recent emergence of important human diseases such as Ebola hemorrhagic fever and highly pathogenic H5N1 avian influenza virus and likely will be important in yet undiscovered and undescribed zoonoses.