2 THE PRINCIPLES OF IMMUNOLOGY 



bacteria in the first few days of extra-uterine life, soon becomes in- 

 habited by large numbers of organisms, which produce no deleteri- 

 ous effect under ordinary circumstances and, in fact, appear to aid 

 in the process of digestion. Animals may adapt themselves to 

 organisms even of the pathogenic varieties, as, for example, in the 

 condition known as the " carrier state," in which virulent diphtheria 

 bacilli or virulent typhoid bacilli are harbored without apparent 

 harm. This capacity is due to certain changes which take 

 place in the body, so that the organisms and their products do no 

 damage. In the carrier state the organisms themselves have prob- 

 ably developed a state of resistance against substances produced in 

 the host which ordinarily would destroy the organisms and neu- 

 tralize their toxic products. 



Parasitism. The parasite is a living organism which carries on 

 its existence within or upon its host, an.d derives its nutrition there- 

 from. Parasitic bacteria include both pathogenic and non-pathogenic 

 forms. Bail has classified bacteria in three forms: (i) Pure sapro- 

 phytes, which do not develop within living animal tissue, but derive 

 nutrition from dead material ; these may be pathogenic, provided 

 they produce poisonous substances which may be absorbed, as is 

 the case with the bacillus aerogenes capsulatus; (2) pure parasites, 

 which live entirely within tissues, including such organisms as the 

 anthrax bacillus; they may exist in a vegetative form for long 

 periods of time outside the body; (3) half parasites, which may be 

 pathogenic if introduced into the animal body, but do not possess 

 the invasive character and the necessity for life within tissues ex- 

 hibited by the pure parasites. Most of the bacteria pathogenic for 

 man belong in this last group, as, for example, the bacillus typhosus 

 and the cholera vibrio. Such organisms as these may live and grow 

 for long periods of time in water, and in foods of various kinds, may 

 vegetate for a certain period under unfavorable conditions, but upon 

 introduction into a susceptible host produce local lesions and in 

 some instances may become moderately invasive for the entire 

 organism. Symbiosis has relatively little significance in human 

 medicine, but certain instances occur, as, for example, the apparent 

 symbiosis of the fusiform bacillus and the spirillum of Vincent's 

 angina. Certain parasitic protozoa, such as the endameba histolytica 

 of dysentery, require the associated presence of bacteria, but these 

 latter are not necessarily pathogenic and the phenomenon is not 

 that of symbiosis, because the endamebse live at the expense of the 

 bacteria, and the organisms, therefore, are not mutually advantage- 

 ous to the existence of each other. 



Virulence. By the term virulence is indicated the capacity of 

 an organism to produce disease. The degree of virulence may differ, 

 not only between different species of organisms, but between strains 

 within the same species. It is probably true also that individual 

 organisms in the same culture possess different degrees of virulence. 

 Furthermore, the virulence of a species or of a particular strain 



