624 THE POPULAR SCIENCE MONTHLY. 



The recent experiments of Nuttall, Behring, Buchner, and 

 others have established the fact that recently drawn blood of 

 various animals possesses decided germicidal power, and Buchner 

 has shown that this property belongs to the fluid part of the 

 blood and not to its cellular elements. This power to kill bacteria 

 is destroyed by heat, and is lost when the blood has been kept for 

 a considerable time, but it is not neutralized by freezing. Further, 

 this power to destroy bacteria differs greatly for different species, 

 being very decided in the case of certain pathogenic bacteria, less 

 so for others, and absent in the case of certain common sapro- 

 phytes. 



In the infectious diseases of man involving the system gen- 

 erally, a single attack commonly confers immunity from sub- 

 sequent attacks. This is true of the eruptive fevers, of typhoid 

 fever, of yellow fever, of mumps, of whooping-cough, and, to 

 some extent at least, of syphilis. But it seems not to be the case 

 in epidemic influenza (la grippe), in croupous pneumonia, or in 

 Asiatic cholera, in which diseases second attacks not infrequently 

 occur. In localized infectious diseases, such as diphtheria, ery- 

 sipelas, and gonorrhoea, one attack is not protective. Croupous 

 pneumonia and Asiatic cholera should perhaps be grouped with 

 diphtheria and erysipelas as local infections with constitutional 

 symptoms resulting from the absorption of toxic products. 



That immunity may result from a comparatively mild attack 

 as well as from a severe one is a matter of common observation in 

 the case of small-pox, scarlet fever, yellow fever, etc., and since 

 the discovery of Jenner we have in vaccination a simple method 

 of producing immunity in the first-mentioned disease. The ac- 

 quired immunity resulting from vaccination is not, however, as 

 complete or as permanent as that which results from an attack of 

 the disease. 



These general facts relating to acquired immunity from in- 

 fectious diseases constituted the principal portion of our knowl- 

 edge with reference to this important matter up to the time that 

 Pasteur (1880) demonstrated that in the disease of fowls known as 

 chicken cholera, which he had proved to be due to a specific 

 micro-organism, a mild attack followed by immunity may be 

 induced by inoculation with an " attenuated virus " i. e., by 

 inoculation with a culture of the pathogenic micro-organism the 

 virulence of which had been so modified that it gave rise to a 

 comparatively mild attack of the disease in question. Pasteur's 

 original method of obtaining an attenuated virus consisted in 

 exposing his cultures for a considerable time to the action of 

 atmospheric oxygen. It has since been ascertained that the same 

 result is obtained with greater certainty by exposing cultures for 

 a given time to a temperature slightly below that which would 



