The host–pathogen interaction is defined as how microbes or viruses sustain themselves within host organisms on a molecular, cellular, organismal or population level. This term is most commonly used to refer to disease-causing microorganisms although they may not cause illness in all hosts. Most of the terminology used to define the host-microbe interaction has been in use for nearly a century. Early in this 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 hosts resulted in the understanding that the host-pathogen interaction does not always result in disease. This recognition, in turn, led to the introduction of terms to explain states in which microbes exist within 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 and conditions 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 of particular microbes, rather than to define a more general host-microbe relationship.