516 Proceedings of Royal Society of Edinburgh. [sess. 
in the part of the district nearest to the hospital ships to indicate 
that the infection was aerial. Any kind of method which would 
result in the transference of the organism would be sufficient, and 
all gross methods are, as all who have had experience in working 
smallpox hospitals know, very difficult to eliminate. The course 
in time is illustrated in the accompanying diagram, placed for 
comparison below that of the corresponding epidemic in London 
(diagrams VIII. and YIIIa.). It will be seen that the period 
of maximum of the theoretical curves very closely coincides, and 
that the general course of the two epidemics is too much alike to 
require the assumption that the Orsett Union outbreak was any- 
thing but the development of an ordinary smallpox outbreak (quite 
possibly due originally to the smallpox ships), resulting from 
the introduction at a definite time of an organism of a definite 
infectivity. 
Conclusions. 
1. An epidemic is an organic phenomenon, the course of which 
seems to depend on the acquisition by an organism of a high grade 
of infectivity at the point where the epidemic starts, this in- 
fectivity being lost from that period till the end of the epidemic 
at a rate approaching to the terms of a geometrical progression. 
2. This loss of infectivity, though realised quite clearly by 
Dr Farr and many other epidemiologists, has not been given the 
importance which is due to it. For instance, in estimating the 
conditions of the spread of smallpox from hospitals, it has been 
assumed that a constant supply of acute cases is necessary, and 
that, though a good number of convalescent cases are removed into 
a hospital, there is little risk from these, without examining 
whether these two factors occurred at different periods of the 
epidemic, when the infectivity of the organism might be greatly 
different. Also, in experiments such as the transmission of plague 
from one animal to another by means of fleas, no regard seems to 
have been paid to the question as to whether the organism was in 
a condition to transmit the infection. Attention has only been 
paid to whether the culture was or was not virulent — a different 
question altogether. Negative results attained in this manner are 
clearly worthless. 
