128 HOUSEHOLD BACTERIOLOGY 
the body machine, which with sinister intent took pos¬ 
session of our interiors and battled for our lives; or 
was a visitation of Providence about which we might 
not inquire too curiously. Then suddenly we became 
aware that the soil, air, and water, the surfaces of 
plants and of our own bodies were swarming with 
minute, invisible, living beings, some few of which 
were of the greatest importance to man because they 
were capable of inciting serious disorders. By a tech¬ 
nical device of the laboratory it was soon found pos¬ 
sible to secure these invisible plants from their various 
sources, to separate them one from another, and to 
cultivate and study them with as much precision as the 
farmer grows and gathers his various crops. 
Of course at first the few harmful members of this 
newly exploited group of living things cast a shadow 
over all the rest. And we shuddered as the pioneer in 
this new domain of science revealed the thousands 
and tens of thousands of bacteria which we might be 
swallowing with our glass of water or with our bunch 
of grapes. But we were soon reassured, for we were 
told that we had nothing to fear from the rank and 
file of our humble, newly discovered commensals; that, 
on the contrary, they were our friends, without which, 
indeed, the world of life could not long continue. It 
was only the few which we must avoid if we would 
steer clear of tuberculosis, pneumonia, diphtheria, ty¬ 
phoid fever, cholera, and a dozen or so others of the 
uncanny brood of infectious diseases. 
