Publishers ' Note 
This book presents in popular form the 
present state of science. It has been reviewed 
by a specialist in this field of knowledge. An 
excerpt from this review follows: 
u This book contains a number of essen¬ 
tial and significant facts told in a simple, 
clear, and interesting manner. It dis¬ 
cusses many things that young people as 
well as their elders should know about 
the small organisms which the human 
race has discovered in the past fifty years 
to have such potent influences on life and 
health and on the momentous changes 
that take place in the world around us!* 
Edwin Oakes Jordan 
Professor of Hygiene and 
Bacteriology 
The University of Chicago 
