CHAPTER I.PARASITISM, INFECTIOUSNESS, CONTAGIOUSNESS.Parasitism is the condition in which a plant, or an animal being, lives on or within another living organism. A true parasite always derives its sustenance from Parasitism. the tissues of its host.Some parasites may live on a host without causing appreciable damage, that is, they are non-pathogenic parasites. In this case they may derive their nutrition from some of the excreted non-living products of the host, living as pure saprophytes,1 or the amount of nutritious substance which they obtain from the host may be so little that the health of the latter is not impaired.There is another large class of organisms, however, which under the proper conditions cause severe diseases in the host. For example, there are many pathogenic microbes which live in and on the skin without doing harm, but if certain ones reach the deeper tissues, they may
Special Article.. JAMA. 1905;XLIV(4):294–295. doi:10.1001/jama.1905.02500310038002
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