46 Transactions. ^Miscellaneous. 



as long as these conditions exist, so long will they flourish and 

 the diseases continue. 



Another question arises — namely, Why should an epidemic 

 finally disappear ? This brings us to the subject of the 

 attenuation of bacterial virus. It may perhaps be possible to 

 stamp out an epidemic on its first appearance, and before it 

 has spread, by vigorous hygienic measures ; but when it has 

 obtained a firm hold in a densely-populated city stamping out 

 becomes practically impossible. Such diseases as cholera, re- 

 lapsing fever, and typhus rage like a wide-spreading fire, and 

 are more difficult to extinguish or limit. A fire will, of course, 

 become extinguished when the inflammable material is all 

 consumed ; but epidemics decline spontaneously without at- 

 tacking all who might be susceptible. I use the word " spon- 

 taneously " provisionally only. It would be difficult to believe 

 that the subsidence of the epidemic was due to increased 

 energy in sanitation, for, to all appearances, the conditions 

 may still exist in many parts of the large towns wliich are 

 usually held to invite zymotic disease. 



The word " attenuation," as applied to the virus of an in- 

 fectious disease, is understood to mean that the germ or bac- 

 terium causing it exists in such a condition that it only 

 produces a poison of mild variety, or that its poisonous pro- 

 perties have been totally extinguished. The most notable 

 example of attenuation is that of the virus of small-pox by 

 inoculation. Inoculation for small-pox was practised in India 

 and the East for centuries before it was known in Western 

 Europe. Introduced into the body in this way the disease 

 was certainly attenuated ; but, several deaths having occurred 

 from the inoculated disease, and vaccination with cow-pox 

 lymph being found to be safe, inoculation was made illegal by 

 Act of Parliament in 1841. It is more than probable that 

 many deaths used to occur from inoculation owing to the want 

 of proper precautions in performing the operation, the neces- 

 sity for which precautions was not then understood. But, not- 

 withstanding the occasionally fatal result of inoculation, it is 

 established that in the great majority of cases the disease was 

 rendered milder than when contracted in the ordinary way. 

 This was an instance of attenuation before the nature of the 

 virus was understood. M. Pasteur, having observed that 

 fowl-cholera in the natural state is not always fatal, concluded 

 that the virus occurred in variable degrees of intensity ; and 

 he noted also that fowls which recovered were not again, or, 

 at least, rarely, subject to a second attack. It occurred to M. 

 Pasteur that by artificially attenuating the virus the disease 

 might be rendered milder. By making successive cultures 

 even of the most virulent micrococcus in chicken-broth at pro- 

 longed intervals, he was able to produce a true "vaccinal" 



