
MY WORK AND BACTERIOLOGY 305 
vated non-virulent varieties of bacilli, resembling 
very much the pest bacillus, from the environs of 
men in houses infected with pest. . . . Pest occurs 
spontaneously in rats. Epidemics of pest in rats 
often precede those in man. It appears as if certain 
tropical soil bacilli first become acclimated to the 
rat’s body, and then are transferred to man.” } 
In ways thus indicated, then, quite independently 
of any de novo origin of micro-organisms, we may 
have such contagious diseases as_ septicaemia, 
typhoid fever, diphtheria, scarlet fever, plague, and 
other communicable affections arising de novo, in 
consequence of the transformation under this or that 
set of local conditions—modifications of ‘ soil” in fact 
—of some common, ever-present micro-organisms. 
From common organisms, they become “specific” 
or pathogenic organisms, and contagious diseases 
are thus engendered, which may spread more or 
less widely among the community. 
It is astonishing, however, how reluctant patho- 
logists are to admit such possibilities—and how they 
still cling, in spite of the most obvious teachings, to 
ultra-contagionist doctrines. 
In conclusion, I will give one notable instance of 
this, touching the origin of a fearfully common 
malady which not many years since was regarded 
as far more frequently having a de xovo than 
1 It is almost universally held by sanitary officers in India, who 
have been engaged in dealing with the recent epidemic, that the 
disease in rats is principally responsible for the spread of plague ; 
and, as a corollary to this statement, all the evidence that has been 
accumulated seems to show that rat-fleas are the chief agents in the 
transmission of the disease. 
U 
