PRESIDENTIAL ADDRESS. 5D 
may be the chief sources of infection, sometimes in large 
numbers, of human beings. In the second place other 
diseases are closely allied to, though probably not 
identical with, similar diseases in man. Conditions 
may exist for working out the full etiological his- 
tories of these latter rat diseases, which may be impossible 
in the case of the allied diseases in human beings. As 
instances of the first-named conditions may be given plague, 
trichinosis, rat bite fever, spirochetal jaundice, and per- 
haps trench fever. An instance of the latter type is rat 
leprosy. 
Commumty Animals and the Origin and Spread of 
Epizootic Disease: the True Significance of Disease.— 
Both rats and mice are, to a great extent, ‘‘community’’ 
animals. In other words, their numbers may be consider- 
able within a limited space, and the individuals must come 
frequently in direct contact with each other, whilst they 
associate together in the same place over long periods of 
time. Such circumstances favour the spread of epizootics. 
It is quite clear that if the members of any particular species 
of animal live in couples widely separated from their neigh- 
bours, there is little chance of noxious organisms passing 
readily from individual to individual of the species. . If the 
organism be rapidly fatal to its host, the victim dies alone, 
and the germs with it. If the host be a community animal, 
if it be a member of a herd, before its death it may have 
conveyed the germs of its disease, either directly or by con- 
tamination of its surroundings, to some of its fellows. 
Further, we know in bacteriology, that the rapid passage of 
certain bacteria from one individual to another tends to en- 
hance their virulence. Such rapid passage is easy of 
achievement in community animals, but more difficult of 
fulfilment the more solitary-living are the hosts. A com- 
munity-living animal, therefore, not only gives a_patho- 
