ABSTRACT
Parasitism is a life-style that has been adopted by many types of living things. In the usage followed in this book, all organisms living in or on the bodies of other organisms and drawing their sustenance from them are considered to be parasites; thus the group includes bacteria and viruses as well as arthropods, worms, and protozoa. In practice the categories into which host-parasite relationships have been divided are somewhat arbitrary and grade one. Some of the difficulty in the terminology used to describe host-parasite relationships is the result of historical accident. If one is to understand host-parasite relationships and the relationship of microbes to disease, one must be aware that microbes normally live on the outer surfaces of plants and animals. Because of competitive exclusion, the most abundant microorganisms among the normal microbial populations colonizing humans play a role in maintaining health. The limitations can best be understood if one considers the fluidity and variety of host-parasite relationships.
