Infectious Agents. 59 



ture {toxalhnmoi, inycoprotciii, bacterial protein). They vary 

 in their composition and modes of operation, some being peculiar 

 to certain kinds of bacteria, others common to several forms. In 

 general they act like albumens foreign to the animal, causing in- 

 tlammatory changes, necrosis of cells and tissues and exciting fe- 

 brile reactions. The dead bodies of the bacteria act in the same 

 way to some degree, their toxic substances being freed only by 

 the death and maceration of the microorganisms. 



A number of bacteria produce pathological changes also by 

 elaboration of acids and gas-forming substances, as sulphuretted 

 hydrogen. 



h\ addition to their chemical action it should be said that 

 mechanical disturbances may be occasioned by bacteria which, in 

 their multiplication, produce masses, perhaps for example ob- 

 structing the blood vessels. 



The sum total of the pathogenic properties of a microbe is 

 spoken of as its virulence. According to the quality of toxine 

 produced and the energy of growth of the microbe in the animal 

 body, there may be recognized gradations and differences of in- 

 tensity of virulence of the various genera, species, strains and 

 individual microbes'; their power of disease production corres- 

 sponding with the same factors. Just as in artificial culture 

 in a number of nutrient media a bacterium will either elabo- 

 rate much toxine and grow rapidly or will produce but little 

 toxine and grow slowly, according to the composition, reaction 

 and temperature of the medium ; so in different kinds of animals 

 there is a variation in capability of growth and toxine production 

 of a given microbe. In adaptation to the conditions of nutrition 

 afforded in a given body microorganisms may in greater or less 

 measure lose the power of grovvth exhibited in some other body, 

 may be altered in their pathogenic power ; this is spoken of as 

 change of virulence by transmission. Such change may manifest 

 itself either as an attenuation or as an intensification of virulence. 

 The oldest known example of attenuation by transmission is seen 

 in the change of virulence of the germ of smallpox ; variola, which 

 in man is severe and marked by pock eruption all over the body, 

 produces in the cow merely a local and mild eruption. In the 

 cow, moreover, even in the first generation, it is permanently so 

 attenuated that after reinoculation in man it gives rise to only 

 a benign local eruption (not a general one). Wide differences 

 of virulence exist between the different strains [growths of the 



