A part of an extended organism that itself has as part a population of one or more infectious agents and that (1) exists as a result of processes initiated by members of the infectious agent population and is (2) clinically abnormal in virtue of the presence of this infectious agent population, or (3) has a disposition to bring clinical abnormality to immunocompetent organisms of the same Species as the host (the organism corresponding to the extended organism) through transmission of a member or offspring of a member of the infectious agent population.