The dual conception that a particular parasite species is limited to a single host species and that a particular host species may harbor but one species of a particular general group is obsolete. Selected examples from among the protozoa, worms, and arthropods support the two following generalizations: (1) A particular parasite will develop normally in as many hosts as provide for it adequate environmental conditions and mode of entrance. The number of hosts in nature ranges from one to many. (2) A particular host may harbor few or many parasites whether they may be far removed or closely related taxonomically, according to the milieu and opportunities for entrance it provides. One host may habor one or many species of even a single genus of parasite. The factors known to bear upon host-specificity are the temperature, age, and size of the host, dosage, the virulence of the parasite, the physiologic state of the host, the immunity mechanism, incompatibility of protoplasm, the food of the host, the sensory faculties of parasites, and the heredity of host and parasite.
Thus host-specificity, like host-resistance and host-susceptibility, becomes interpretable in terms of known physiological phenomena, and does not require any special interpretation involving a peculiar quality which species (host or parasite) possess that is separate and apart from what is observable in individuals.