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 saprophytes.
In the infectious diseases of man involving the system generally, a single attack commonly confers immunity from subsequent 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, erysipelas, 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 acquired 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 infectious diseases constituted the principal portion of our knowledge 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
