1. Department of Veterinary Science, University of Wisconsin, Madison, Wisconsin 53706, USA
2. Republished posthumously. Presented as the Keynote Address at the 1969 Wildlife Disease Association Annual Conference, Ames, Iowa, USA, and reprinted in its entirety from Hanson (1969, Bulletin of the Wildlife Disease Association 5: 150–156) in honor of the author's many contributions to the study of wildlife diseases.