266 MICROBES AND TOXINS 



munises, whereas a virulent organism kills before immunity 

 has time to develop. It is often necessary to find inter- 

 mediate stages of virulence. This is what Pasteur perceived, 

 and hence arose his two vaccines against anthrax. 



It is possible to vaccinate with viruses which one handles 

 without actually knowing their nature, with cultures of bacteria 

 virulent or attenuated, and with bacterial products freed from 

 the living microbes. 



VACCINES 

 I. Vaccinations with Unknown Viruses. 



It is not necessary actually to know the microbe of a 

 disease in order to immunize against it. The vaccinations 

 with unknown viruses are, as a matter of fact, among the most 

 perfect known to medicine. 



Small-pox. The microbe of small-pox is unknown, yet 

 preventive inoculations have long been carried on. There 

 exist two methods, both empirical in origin ; the older of the 

 two has been entirely given up in favour of Jenner's method 

 of vaccination. Inoculation or variolisation consisted in 

 inoculating the virus of small-pox itself, so as to produce one 

 or more cutaneous pustules, the development of which saved 

 the body from a general attack. Small-pox acquired in the 

 natural manner invades the whole body : the virus may enter 

 perhaps by the tonsils or the respiratory passages, gaining the 

 blood stream from these and thence reaching the skin and 

 mucous membranes. Once the eruption has appeared it is 

 too late to intervene. When the natural virus is inoculated 

 artificially on the skin, it may give rise to a general infection 

 later on, but in general the first local pustules produce 

 rapidly sufficient immunity to prevent such a general invasion ; 

 but not always; occasionally inoculation results in a fatal 

 attack of small-pox. The virus inoculated was by no means 

 constant, and it was always a case of working in the dark. 

 A correct and witty account of the history of inoculation is to 

 be found in the eleventh of Voltaire's Lettres Philosophiques. 



