General Principles

Publisher Summary This chapter provides an overview of various interspecies relationships, such as parasitic relations, symbiotic relations, and commensalism. However, the chapter is mainly concerned with the parasitic associations and pathogenicity of several microorganisms. Generalized infections, such as measles, tuberculosis, or typhoid are clearly examples of parasitism. A successful parasitic microbe, like all successful parasites, tends to get what it can from the infected host without causing too much damage. The concept of balanced pathogenicity is helpful in understanding infectious diseases, but many infections have not yet had time to reach this ideal state. It is the virulence and pathogenicity of microorganisms, their ability to kill and damage the host that makes them important to the physician or veterinarian. As molecular biological and immunological techniques are brought to bear on these problems, knowledge is steadily increasing beyond descriptive pathology to a more detailed understanding of host pathogen interactions at the cellular, genetic, and biochemical levels.