INTRODUCTION 19 
tioii, however, to the fact that certain diseases are pecuHar to trees, to 
animals, or to man. Also, some diseases affect the young, but not the 
aged; one sex and not the other. Ophthalmia affects onl\' the eye. 
Finally he recognizes indi^'idnal immunity; some men walk unharmed 
amid pestilence while others fall. 
Anton von Leeuwenhoek, a Dutch linen draper who spent his spare 
time making lenses, appears to have been the first to see bacteria. In 
1675, with lenses of his own grinding, he examined various putrescent 
fluids, drops of water, scrapings from his teeth, and his own diarrheal 
discharges. He says in his writings,^ collected and edited by Robert 
Hooke,- "With great astonishment I observed everywhere through the 
material which I was examining, animalcules of the most minute size, 
which mo\-ed themselves about very energetically." It is possible to 
recognize cocci, bacilli and spirilla in his drawings,^ and it is almost 
certain that he actually observed motility among his organisms. The 
learned monk, Athanasius Kircher, observed and described "minute 
living worms" as early as 1659, but his optical equipment was inferior 
to that of von Leeuwenhoek and it is doubtful if he actually saw 
bacteria. 
Improvements in the microscope opened a new world for in^'esti- 
gation, and speculations concerning the doctrine of the Spontaneous 
Generation of Life led to numerous investigations of increasing refine- 
ment that finally resulted in the brilliant researches of Pasteur and 
Tyndall, who showed by numerous ingenious and carefully executed 
experiments that the phenomena in putrescible fluids erroneously 
interpreted as spontaneous generation did not take place when proper 
precautions in manipulation were observed. About 1835 achromatic 
lenses for the microscope reached a state of perfection compatible 
with the examination of minute objects, and the microscope was 
almost immediately applied to the study of various morbid processes, 
with remarkable success. 
Bassi (1837) discovered a fungus which caused a contagious disease 
of silk worms known as muscardine; Cagniard de Latour and Schwann 
observed and described the yeast plant in liquids undergoing alcoholic 
fermentation. 
Ehrenberg (1838) published his classification of animalcules and in 
his group of Vibrionia described several "species" of organisms, as 
follows : 
1. Bacterium— rigid and filamentous organisms. 
2. Vibrio— flexuous and filamentous organisms. 
3. Spirillum— rigid spiral filamentous organisms. 
4. Spirochseta- flexuous spiral filamentous organisms. 
This classification, which contains terms widely used in bacterial 
nomenclature today, was followed in 1872 by the important con- 
1 Arcana Naturae Detecta, published by Joh. Arnold Langerak, MDCCXXII. 
2 Collected Memoirs of Anton v. Leeuwenhoek, Royal Society of London, 1675, 1683. 
' See ref. 1, p. 40 for these drawings. 
