De Zouche. — Bacteria and their Belation to Disease. 47 



virus, which being inoculated into fowls not only does not kill 

 but actually protects from the fatal form of the disease. 

 M. Pasteur communicated these results to the Academy in 

 1880. 



Anthrax, also, has been attenuated by M. Pasteur so that 

 he was able to produce virus of different degrees of virulence — 

 a virus which produced anthrax in sheep, cows, and horses, 

 but did not cause death, while it was still fatal to guinea-pigs 

 and rabbits, and, finally, a very attenuated virus, which had 

 lost its virulence for guinea-pigs and rabbits, and protected 

 them against an attack. Further, he was able to keep up the 

 artificial cultivation of the microbes, now rendered inoffensive. 

 But an important exception must be made in the case of 

 guinea-pigs, for the attenuated virus was found to be fatal to 

 a guinea-pig one day old, and the virulence could again be 

 restored by inoculating a series of guinea-jDigs, until in the end 

 it was strong enough to kill sheep, and M. Pasteur does not 

 hesitate to say that it would kill even cows and horses ; or, as 

 M. Koux expresses it, the bacillus can be made to reascend the 

 steps of virulence down which it has come, and so be rendered 

 once more virulent. Swine-plague virus is fatal to rabbits, and 

 the virulence increases a hundred-fold by passing it through a 

 series of rabbits. But, according as it becomes most deadly 

 to rabbits, it also becomes attenuated for the pig, and may be 

 used to protect that animal from the disease. 



Some of the questions regarding the cause of epidemics and 

 contagions and their virulence or mildness are thus treated 

 by M. Pasteur. He says, "The above facts may help to ex- 

 plain the appearance of these plagues. An epidemic which 

 has been extinguished by the weakening of its virus may light 

 up again by the strengthening of this virus under certain in- 

 fluences. The accounts which I have read of the spontaneous 

 appearance of the plague appear to me to offer examples of 

 this : witness the plague at Benghazi in 1856-58, where the 

 outbreak could not be traced to any origin by contagion. 

 Plague is a virulent disease peculiar to certtiin countries. In 

 all those countries its attenuated virus must exist, ready to 

 take on its active form whenever certain conditions of climate, 

 of famine, of poverty again appear. There are other infective 

 diseases wliich appear spontaneously in all countries : such is 

 the typhus of camps. Without doubt the germs of microbes, 

 the authors of these diseases, are spread about everywhere, 

 . . ready to become dangerous when, under conditions of 

 overcrowding, and of successive development on the surface of 

 wounds in weakened bodies, or otherwise, their virulence be- 

 comes progressively strengthened. . . . What is a micro- 

 scopic organism which is innocuous to man or to some particular 

 animal ? It is a being which cannot develope in our body or in 



