48 INFECTION AND IMMUNITY. 



etc., than they could reach the lungs through in- 

 fected dust or droplets. Similarly, if the virus of 

 scarlet fever, which is excreted from the lungs and 

 skin, could cause infection only through cutaneous 

 wounds, rather than through the air passages, there 

 is reason to believe it would not hold its pres- 

 ent position as a very contagious disease. Among 

 the communicable diseases we have to recognize 

 that each has its own mechanism for habitual 

 transmission, although the habitual mechanism 

 may be departed from on many occasions. 

 Mediums of As would be supposed, the diseases which are 

 reyance. mog rea( jjiy acquired, the most contagious, and 

 the most prevalent are those in which the micro- 

 organisms are excreted into the air from the res- 

 piratory passages, and in which also the respira- 

 tory passages are the preferred infection atrium. 

 The medium of conveyance i. e., the air is used 

 alike by all individuals. Some other diseases, much 

 less contagious than those mentioned, may pre- 

 vail in extensive epidemic form, often exceeding 

 scarlet fever, measles, etc., in the percentage of 

 incidence, as in the cases of cholera and typhoid 

 fever. This is commonly due to an infected water 

 supply, and the distribution of the disease cor- 

 responds in large degree to that of the contami- 

 nated water. In other words, the epidemic is co- 

 extensive with the use of the conveying medium. 



Similarly, the prevalence of gonorrhea and syph- 

 ilis depends on the extent of promiscuous sexual 

 intercourse; and, of malaria and yellow fever, on 

 the number of individuals who become bitten by 

 the infected Anopheles or Stegomyia mosquitoes. 



Thus the central factor in the communication 

 of infection is suitable contact with the agency 



