Epilogue

Abstract

In the preceding discussion (Chapter 26), centering largely on the epidemiology of a number of parasitic infections and how this is affected by various ecological conditions, I have considered parasite populations in terms of their average distribution in a given population of hosts. I have ignored the salient fact that parasite ecology involves exploitation of a small patch of resources, the individual host. Each host is like an island, separated by a smaller or greater expanse of sea, the hostile external environment. The patchy resources and their ephemeral nature all contribute to the nonequilibrium conditions in which parasites exist. They typically have high rates of reproduction with strong tendencies to parthenogenesis and hermaphrodism. They show rapid rates of evolution with often polytypic species. Much of the evolutionary history of parasites has been determined by the development of different ways to get from island to island, i.e., from host to host. This brings us back full circle to the beginning of this book where I discussed the establishment of infection. We are just beginning to understand the physiological and develomental mechanisms, the cellular and molecular interactions involved in getting out from one host and into another. This remains as one of the most fascinating and most important fields for future inquiry. With intracellular protozoa, receptors on their surface evidently interact with molecules on the surface of the host cell.