IMMUNITY AND SUSCEPTIBILITY. 115 



b. I'accination. — This word, derived from vacca, a cow, 

 had its origin in the use of matter from the pustules of 

 cowpox, asjenner used it to prevent smallpox. In its 

 etymologic sense it is not strictly applicable as now em- 

 ployed, but it has become the convenient designation of 

 all modified "viruses'''' or cultures of pathogenic bacteria. 



The vaccination against smallpox depends upon an 

 attenuation of the variola germ as it passes through the 

 cow, by which its energies in man are limited to the 

 development of a local lesion and mild constitutional 

 involvement, devoid of all contagiousness. The essence 

 of the process is the attenuation of the germ in the cow. 

 Laboratory experiments have enabled us to produce vac- 

 cines to many diseases by manipulating their germs so as 

 to destroy their pathogenic powers without limiting their 

 immunizing powers. Pasteur 1 found that if the Bacillus 

 anthracis was grown at certain temperatures, it lost its 

 virulence, so that it failed to kill animals larger than 

 mice. He also observed that the inoculation of these 

 attenuated germs into the cow was followed by no im- 

 portant symptoms, though the cow became resistant to 

 more virulent cultures. By a second inoculation of a 

 vaccine or attenuated culture fatal for guinea pigs, and 

 then a third, fatal to rabbits, the cow attained a perfect 

 protection against infection with virulent anthrax. 



Vaccination against symptomatic anthrax has been sim- 

 ilarly accomplished by Arloing, Cornevin, Thomas, 2 and 

 Kitt, 3 who found that if the bacilli, dried in powdered 

 muscle from affected animals, were exposed for some hours 

 to a temperature of 85 C, they became attenuated and 

 no longer pathogenic for cattle, though their inoculation 

 into them was succeeded by perfect immunity. HafT- 

 kine* has successfully used modified cultures of the chol- 



1 Compte-rendu de la Soc. Biol, de Paris, 1881, xcii., pp. 662, 665. See, also, 

 vol. xc. 



* Le Charbon Symptomatique du Bceuf., Paris, 1887. 



* Centralb. f. Bakt. u. Parasitenk. , i. p. 684. 



* Brit. Med. Jottrn., 1891, ii., p. 1278; 1895, ii., p. 1541. 



