20 INFECTION AND IMMUNITY. 



er. The micro-organisms are excreted mainly by 

 the feces, and typhoid and paratyphoid by the 

 urine. As a rule they are acquired indirectly, as 

 through a water supply or milk which have been 

 infected from discharges, contamination of the 

 hands from the excreta to food. Examples are ty- 

 phoid, paratyphoid, cholera and dysentery. 

 insects. 4. Transmission by means of insects. Some of 

 these diseases, as malaria, yellow fever, and Rocky 

 Mountain spotted fever, are not contagious at all, 

 but are nevertheless communicable through the 

 medium of the proper insect, mosquitoes or ticks. 

 South African tick fever, ordinary relapsing fever, 

 plague in some instances, trypanosomiasis, and 

 probably typhus are other examples. The steps 

 in transmission are sometimes very complex, and 

 vary a great deal, as will be pointed out later. 

 Not Trans- 5. Transmission from man to man does not 

 missibie. ^ e pj ace a ^. a j} un( j er ordinary conditions: tet- 

 anus, hj'drophobia, and other wound infections. 



It is thus seen that these five divisions consti- 

 tute a series in which contagiousness finally disap- 

 pears. The subject of transmission will receive 

 further consideration later. 

 infections It has been stated above that infectious diseases 



Siilistances. TIT- j/i T 



are caused by living pathogenic organisms. In- 

 " vestigations have shown, however, that the toxic 

 products of some organisms can be prepared and 

 separated from the organisms themselves by filtra- 

 tion, and that such microbe-free toxins when in- 

 jected into animals may cause the same symptoms 

 that are produced by the bacteria themselves (teta- 

 nus and diphtheria) . Accordingly, for the sake of 

 convenience, these toxins also may be considered 

 among the infectious agents, even though sepa- 



