10 Transactions Texas Academy of Science. 
The pronounced success of vaccination gave great impetus to the 
investigation of the problem of immunity, and the annals of the 
nineteenth century contain a voluminous record of the prolonged 
and patient efforts of a host of brilliant workers whose contributions 
have at least laid the foundation upon which the solution of the 
problem may, in the future, be built. The building of this foun- 
dation can not be recounted here ; but it will be necessary to men- 
tion some of the materials of which it is made that the latest pro- 
gress may be intelligently discussed. 
As in the case of smallpox, it had long been a matter of common 
observation that a number of the acute infectious diseases occur 
but once in the same individual. Whooping-cough, measles, scar- 
let fever and yellow fever are notable examples of acute infectious 
diseases, one attack of which usually confers immunity against sub- 
sequent attacks of the same disease. It was also observed that some 
infectious diseases confer a very evanescent type of immunity, and 
that others confer no immunity whatever. 
From the standpoint of immunity the infectious diseases may be 
easily divided into three classes : 
1. Diseases one attack of which confer immunity against sub- 
sequent attacks of the same disease. 
2. Diseases one attack of which confer immunity against sub- 
sequent attacks of the same disease for only short periods of time. 
3. Diseases an attack of which confer no immunity whatever. 
It would seem that these facts, coupled with Jenner’s discovery 
of a fundamental and practical method of producing artificial im- 
munity, clearly outlined the path for f uturh workers to follow ; but, 
strange to say, the nineteenth century was well on its way before 
this important route found many followers. 
The failure to fully appreciate Jenner’s brilliant discovery, and 
to apply his method to the study of other infectious diseases, finds 
an explanation in the hazy theoretical conceptions of the cause and 
nature of infectious diseases which prevailed during the early part 
of the century. The investigations of fermentation by Astier, 
Sette, Franz Schulze, Cagnaird de Latour, Schwann, Fuchs, 
Eemak, Mitscherlich, Helmholtz, and others, did much toward 
clearing the haziness of that period; but it was the monumental 
work of Pasteur that “finally established the truth of the view that 
all processes of fermentation and putrefaction alike are caused by 
living things, and that in each different fermentation different 
kinds of microbes are concerned.” 3 In the light of newer knowl- 
edge this statement needs revision. The investigations of Koch 
