Vol. 68, No. 12
Host-PathogenInteractions: Basic Concepts of Microbial Commensalism, Colonization, Infection, and Disease
Division of Infectious Diseases, Department of Medicine, andDepartment of Microbiology, Albert Einstein College of Medicine, Bronx, New York 10461 Most of the terminology used to deﬁne the host-microbe interaction has been in use for nearly a century. Early inthis period, microbes were thought to be primary aggressors that governed the host-pathogen interaction, resulting in disease. Later, new information about the attributes of microbes and their hostsresulted in the understanding that the hostpathogen interaction does not always result in disease. This recognition, in turn, led to the introduction of terms to explain states in which microbes existwithin hosts without causing overt disease and why some microbes only cause disease in certain hosts. Commensal, carrier state, and opportunist were terms put forth to account for microbes andconditions that were sometimes associated with disease but for which Koch’s postulates could not be fulﬁlled for one reason or another. Most of these terms were originally proposed to describe the behavior ofparticular microbes, rather than to deﬁne a more general host-microbe relationship. Recently, we reviewed the concepts of virulence and pathogenicity and described how the deﬁnitions for these termschanged over the years as microbiologists tried to ﬁnd ways to convey that microbial pathogenesis reﬂects an interaction between two entities, host and pathogen (7). Based on the concept that hostdamage was the most relevant outcome of the host-pathogen interaction, we proposed revisions to the deﬁnitions of the terms pathogen, pathogenicity, and virulence (7). However, the proposed framework...