INTRODUCTION. 21 
lished his views in 1762, maintained that not only 
were all infectious diseases caused by micro-organisms, 
but that the infective material could be nothing else 
than a living organism. On these grounds he en- 
deavored to explain the variations in the period of 
incubation of the different infectious diseases. He also 
insisted that there were special germs for each infectious 
disease by which the specific disease was produced. 
Plenciz believed, moreover, that these organisms were 
capable of multiplication in the body, and suggested 
the possibility of their being conveyed from place to 
place through the air. He also made original investi- 
gations into the process of decomposition, and having 
found “animalcule” in all decomposing matter, he 
became so thoroughly convinced of the causative rela- 
tion of these organisms to the process that he formu- 
lated the law that decomposition takes place by means 
of living organisms, and is possible only through their 
increase. 
These views, it is true, were largely speculative, and 
rested upon insufficient experiment; but they were so 
plausible, and the arguments put forward in their sup- 
port were so logical and convincing, that they continued 
to gain ground, in spite of considerable opposition and 
ridicule, and in many instances the conclusions reached 
have since been proved to be vorrect. The fact that 
infectious diseases were of sudden occurrence, breaking 
out often in isolated places, and that they frequently 
remained clinging for long periods to certain localities, 
leaving others unaffected, was evidence that they were 
not produced by a gaseous infective agent. Moreover, 
the mode of infection, its unlimited development among 
large numbers of individuals, and gradual spread over 
