INFECTION AND IMMUNITY. 167 



ance to the incursions of the germs which the body cells at different times 

 and under differing conditions exhibit, are all factors of the greatest 

 moment. 



Malnutrition, mental or physical overwork, injuries, bad hygienic 

 surroundings, the abuse of alcohol and other forms of intemperance, as 

 well as many other conditions which lead to deterioration in the general 

 health, are often decisive factors in determining the intensity or even 

 the occurrence of the diseases due to bacterial incitement. It is thus 

 clear that the action of a given germ in the living body depends only in 

 part upon its intrinsic capacities which in themselves are very variable 

 but also and in marked degree upon the capacities, also variable, 

 which exist at the moment in the body cells among which the lot of the 

 germs is cast. 



It should be always borne in mind that the human body is a great 

 aggregate of groups of co-ordinated cells which, under normal conditions, 

 all act in harmony for the maintenance of the life and functions of the 

 individual. The cells and cell communities in health not only do this, 

 but they have the power of resisting and to a certain extent overcoming 

 various deleterious agencies to which the body is more or less constantly 

 liable. 



What we call hereditary or acquired predisposition to an infectious 

 disease, such as tuberculosis for example, is simply a lack of the usual 

 capacity of the cells of the body whether through a structural or physi- 

 ological fault we do not yet know to cope with the destructive tenden- 

 cies of the living micro-organisms when once these gain a foothold in the 

 body. 



We thus see that, in studying the conditions under which infectious 

 diseases occur, the work is by no means complete when the bacterial 

 species which incites the disease has been discovered, but that then the 

 more obscure determining and influencing agencies must be worked out 

 in each case. 



Infection and Immunity. 



INFECTIOUS DISEASE AND THE NATURE OF INFECTION. 



Infectious diseases are those which are incited by the entrance into the 

 body and proliferation there of pathogenic micro-organisms. Infection is 

 the act or process by which such diseases are incited. 



In the more exact usage of the words infectious and infection which 

 our new knowledge demands, it is customary and convenient to limit the 

 term micro-organism to the fungi bacteria, yeasts, and moulds and to 

 the protozoa representing the animal kingdom, excluding altogether the 

 eutozoa and other animal parasites. ' 



1 For a resume of the nature and conditions of infection, see Wassermann, Kolle and 

 Wassermann's "Handbuch der Mikroorganismen,"-Bd. i., p. 223, Bibl. ; for a study of 

 the general reaction of the body in infection see Blumenthal, ibid., p. 326. 



' 2 With this somewhat arbitrary limitation, neither trichinosis nor scabies, for exam- 

 ple, would be considered an infectious disease, although they are often, for the lack of 

 a distinctive word, thus designated. 



