116 INSECTS AND DISEASE 
tained abundantly in the sputum, and these cases are 
extremely infectious, as was shown in the small outbreak 
in Maryborough, in 1905. But in bubonic plague, and 
even in the septicaemic form, there does not appear to be 
more than an occasional and trivial exit of the bacillus, 
and in these cases, which form the vast majority, plague 
has been found by experience to be non-infectious. 
It has been known for a long time that true plague 
is not a disease limited to the human species. Not only 
can many, we might say most or all mammalia, be infected 
by artificial inoculation, but the disease has in many species 
occurred under natural conditions. Especially is this the 
case with rodents, and among them the species of Mus that 
are associated with mankind are affected above all others. 
It has been found in Australia that epizootics of rat-plague 
have accompanied and preceded outbreaks of human 
plague, and this has been so also in many other parts of 
the world. It has been the case so uniformly, wherever 
adequate research has been made, as to suggest a causal 
connection between the epizootic and the epidemic. That 
is to say that plague is primarily a disease of the rat, com- 
municated from rat to rat, and incidentally communicated, 
when the conditions are favourable, from rat to man. 
Now it is obvious that the conditions under which plague 
spreads naturally from rat to rat are open to experimental 
investigation, and if these were satisfactorily established, 
much light might be expected to be thrown on the occurrence 
of the disease among mankind. 
The first direct experimenta] evidence as to the natural 
method of rat infection was obtained in Bombay, in 1898, 
by Simond, who showed that plague could b> conveyed 
from one rat to another, not allowed to come into contact 
with it, provided fleas were allowed to pass from the infected 
to the healthy rat. He observed that on rats suffering 
from natural plague fleas were usually abundant, and that 
the fleas that left a rat which had died of plague contained 
virulent plague bacilli. Plague in man, he attributed to the 
infected persons having been bitten by fleas which had 
left a plague rat. This conclusion was strongly supported 
by an epidemiological study of the epidemic in Bombay, 
by Hankin, published in the same year. This rat-flea 
