DOSAGE OF MICRO-ORGANISMS. 93 



inhabit the mucous surfaces constantly, it would 

 seem that an adaptation has taken place between 

 such surfaces and the micro-organisms, so that the 

 latter, although often pathogenic, are not able to 

 reach deeper tissues. The least harmful infection 

 of which one could conceive would be one in which 

 the micro-organism disturbed the host in no way, 

 except in so far as it used a certain amount of its 

 substance for nutrition. This would be a case of 

 simple, comparatively harmless, parasitism, which 

 in man, is best illustrated by the organisms which 

 live habitually on the mucous surfaces. 



It is possible that harmless tissue invasion finds 

 examples in some of the insects, although this may 

 not be maintained positively. The apparent harm- 

 lessness of the organisms of malaria, yellow fever, 

 South African tick fever and Rocky Mountain spot- 

 ted fever for the insects which harbor them, was 

 mentioned in the preceding chapter. Certain infec- 

 tions do, indeed, run a very chronic and benign 

 course, as the ordinary trypanosomiasis of rats, but 

 they are not without discoverable, or even fatal, 

 effects eventually. 



The quantity or dosage of micro-organisms re- Dosage. 

 quired for infection depends on their virulence 

 and the degree of their parasitic power (infectiv- 

 ity) ; this varies with different species and also 

 with different strains of the same organism. The 

 anthrax bacillus is very infective and may reach a 

 high degree of virulence, so that a single organism 

 has been known to produce fatal disease in experi- 

 mental animals. Extremely minute quantities of 

 some of the more virulent trypanosomes are 

 required. The tubercle bacillus, even when most 

 virulent, is hardly so infective ; it is said that eight 



