﻿6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 59 



asites, such as those causing cholera and dysentery, are not stayed 

 by these conditions, but are themselves capable of producing damage, 

 although being assisted by other injurious agencies. 



At first sight, it appears paradoxical that very young infants should 

 show, in view of these facts, a definite resistance to such a disease as 

 measles. But the discrepancy finds an easy explanation in the im- 

 munity acquired from the mother, by passive transfer of the protective 

 principles, first from the blood of the mother during pregnancy 

 and later with the milk during nursing. This immunity, being but 

 passive, soon disappears, and the child of a few years then becomes 

 readily infectible. The milk is the one secretion that derives from 

 the blood the immunity principles in large amount, in this differing 

 from all other glandular secretions ; and here the coincident permea- 

 bility of the intestine in the young scores an advantage against the 

 parasites, since they are absorbed from this source into the blood. 



Not all parasites of the same species, whether bacterial, protozoal, 

 or submicroscopic, are potentially equal as agents of infection. The 

 quality of virulence, so-called, is of high importance. Not a few of 

 the more common parasites vary greatly in virulence, from degrees 

 that make them almost harmless to degrees that make them almost 

 inconceivably potent. Many millions of non-virulent pneumococci 

 may fail to affect a rabbit, and a single virulent pneumococcus may 

 suffice to produce certain death. This state of virulence is determined 

 in some instances by races of the parasite of particular quality, so 

 that what is virulent for one animal is not necessarily virulent for 

 another. Thus the pneumococcus culture, of which a single germ 

 will set up fatal blood poisoning in the rabbit, may fail utterly, even 

 when many millions are injected, to affect the far smaller guinea pig. 

 Such races of the poliomyelitis virus have now been recognized. The 

 human strain, so called because it is derived originally from human 

 cases of the disease, has at first small power to cause infection in 

 monkeys. By successive transfer from monkey to monkey, certain 

 samples can be made to acquire a prodigious activity for them, due 

 to conversion, by adaptation, into the monkey strain, which there 

 are reasons for believing has now lost power to infect human beings. 



These changes take place sometimes slowly and sometimes quickly ; 

 in the latter instance, they correspond to or suggest the appearance 

 of " sports " or " mutants " among the higher animals or plants. 

 They have doubtless a definite relation to the prevalence of epidemics 

 of disease, the laws regulating which have still to be worked out. 

 But most epidemic diseases exist at other periods as sporadic dis- 



