Abstract
Infectious diseases can be manifested by a spectrum of clinical signs and resultant clinical courses that range from acute to chronic and possible persistence in the victim in a latent form. Until recently, the origins of this kind of biodiversity were poorly understood, but advances in immunology — especially in identifying the constitutional mechanisms of immunity — have contributed to our understanding of the origins of biodiversity in infectious diseases. Infectious diseases affect only focal areas in the affected organisms, and the amounts and distribution of infectious lesions vary from patient to patient. In a population attacked by an infectious agent, individuals can be conveniently divided into three categories: totally resistant organisms which contain no susceptible structures and are not affected; mildly susceptible organisms in which a few foci appear and in which the infection runs a benign course; organisms in which the number of constitutionally susceptible structures is high and the infectious process develops in a severe form. The diversity is determined by the differences in susceptibility of various parts of the organism under consideration.