ABSTRACT
Important insights are gained by looking into the evolutionary selection pressures and host-microbial niches to which class I and class II pathogens have evolved (Figs. 1 and 2). In general, two distinct but overlapping physical, temporospatial, and host defense-related niches are observed for any given species, which pathogenic and nonpathogenic microbes might exploit. The first niche is the prenatal/early postnatal period, and given that the most basic tenet of life is reproduction, any given species in any given year will produce some number of offspring or face extinction. Thus, a supply of newborns and therefore immunologically naı¨ve offspring will be available for colonization and/or infection. Other thanmaternal antibody (colostrums, egg yolk, etc.), the evolutionary equivalent of maternal vaccination and passive transfer therapy of the newborn, the early postnatal period is a rather vulnerable period of time in the host, thus providing a well-defined cellular/humoral niche in which to evolve. As the maternal antibody wanes or an unfortunate offspring fails to receive adequate colostrum, the young host generally experiences increased susceptibility to infection, prior to the induction of any specific acquired immunity. Incumbent on the successful evolution of class I pathogens (in a relatively naı¨ve host immune system), would be to invest in a more stable genetics, with high replication/fitness, contagiousness, and efficient transmission profiles rather than investing largely on immune evasion strategies (2,3). The end game of class I pathogens appears to exist in the environment of available offspring by infecting young hosts efficiently, thus amplifying and spreading to new hosts in shorter time periods than class II pathogens.
