Although the foundation of Koch's postulates, that "if an agent is the cause of disease in one individual it should be capable of causing disease in a second individual," is basically sound, the ritual that has evolved into present day experimental studies has obscured almost completely what occurs in natural processes outside the laboratory. Through a series of examples, it is emphasized that just bringing the host and the parasite together is not enough, but that the circumstances under which this is done is equally important. These circumstances include: the prior history of the host; the host's behavioral patterns, environmental conditioning, and disease history; the circumstances of exposure; and the environmental factors related to the host and the parasite. Of equal importance is the individual variation (genetic, physiologic, immunologic, etc.) of the host and the individual variation (strains, immunogenicity, pathogenicity, virulence, etc.) of the parasite. Because the rigor of the present day "scientific method" demands clearcut and reproducible results and investigations require predictable performance of the parasite in an evenly maintained host that is in a highly constrained environment, we should not wonder why we cannot produce the events of nature. If we are going to understand diseases of wildlife, we must consider the genetic heterogenicity of the host and parasite population, and recognize the complexity of the environment in which both exist. Koch's postulates, in the narrow sense, will help us to identify parasitisms but will not provide us with an understanding of information about diseases in wildlife; the real significance of these parasitisms to the health of the individual and to the size of the population.(ABSTRACT TRUNCATED AT 250 WORDS)