1905-6.] Studies in Immunity : Theory of an Epidemic. 515 
Liverpool is identical with that distribution seen in epidemics 
where there is no special local predisposing cause (London, 1902). 
The incidence of cases in the neighbourhood of the Park Hill 
Hospital seems, however, in the Liverpool epidemic a more marked 
example. This case is of very special interest, as a local secondary 
epidemic occurred in the immediate vicinity of the hospital, 
beginning in a marked manner six weeks after the acute cases 
were admitted there. Here two points are again to be noted. 
Cases of smallpox had occurred in this area prior to the opening 
of the hospital ; and though these were few in number, there is 
absolutely nothing in the course of the subsequent epidemic to 
indicate that they were not the sufficient cause. The course in 
time is a typical distribution of type IV.,* which has been found 
to be the general epidemic form. Had special modes of infection 
played more than a subsidiary part in the development of this 
wave, there would seem to be a probability that this would not 
have been the case. There is, therefore, no need to assume 
special modes of infection. In the second place, the epidemic is 
so distributed in space as to have its centre three-quarters of a 
mile from the smallpox hospital ; and if this point be joined with 
the centre of the hospital, it is seen that in the two quadrants 
which are adjacent to the hospital there are few more cases than 
in the quadrants which are remote. Here, again, the fallacy of 
neglecting the real distribution of the cases in space is capable of 
leading to quite untrustworthy results if the incidences in zones 
round the hospital are alone considered. So far as can be judged 
from the map, there is no greater population difference in the dis- 
tribution of the population in the circle of one-quarter of a mile 
radius round the theoretical centre of the epidemic, and apparently 
little in the half-mile zone, while the latter radius includes practi- 
cally the whole of the cases which strictly belong to this outburst. 
Another Local Government Board report has recently been 
issued regarding the apparent aerial spread of smallpox in the 
Orsett Union District from the smallpox hospital ships in the 
adjacent part of ffhe river Thames. 
Here, again, when the space and time distributions are examined, 
there is nothing beyond the fact that the outbreak had its origin 
* Table A, No. 16. 
