ON THE BACTERIA. 163 



attacked by a parasitic mould, which never advances beyond a very 

 simple condition on the flies in the air ; but should an infected fly 

 fall into the water, a much more elaborate development is given to 

 the mould ; and should the fly come in contact with an unhealthy 

 salmon, it may develop into the pathogenic fungus which is the 

 cause of salmon-disease. 



Dr. Roberts, of Manchester, has suggested that disease germs 

 may be modifications or " sports " from harmless Saprophytes, 

 which, from alterations in their surroundings, have acquired a para- 

 sitic habit, and so become dangerous to life. This idea, and 

 Pasteur's discovery, that the attenuation of the virus depends 

 upon the presence of oxygen, leads to the conclusion that various 

 alterations in the "environment," the deprivation of oxygen, or 

 the cultivation in gaseous mixtures from which the normal supply 

 of free oxygen in air is absent, may have an influence in converting 

 harmless germs present in the atmosphere into the dangerous 

 Bacilli of disease ; and that we should not so much try to prevent 

 infectious diseases by bringing into vogue new methods of vacci- 

 nation, but rather combat them by extended sanitation, fighting 

 diseases outside, not inside the body. May it not explain the 

 efiicacy of isolation, the utility of oxidising disinfectants, the 

 salubrity of the country, contrasted with the unhealthiness of 

 towns ; the success of cool or open-air treatment in certain cases 

 of illness, and the decline of zymotic diseases before the progress 

 of sanitation ? 



Dr. Angus Smith has further suggested that the putrefying 

 process, when carried on in confined places, as sewers, may 

 cause a development of disease germs which does not take 

 place when the same processes go on in unconfined places. 

 Analogous conditions may exist in the lungs of persons living 

 in vitiated atmospheres, in ill-ventilated rooms, or engaged in 

 sedentary occupations, not taking sufficient exercise, or of feeble 

 respiratory habit. Innoxious germs in the atmosphere may be 

 inhaled and retained in the lungs of such persons, and then by 

 successive culture and deficient aeration acquire a parasitic or 

 deadly character. This may account for the development of 

 typhoid and other fevers, and of consumption in in-sanitary build- 

 ings, and also for the benefit obtained by consumptive patients by 



