1905 - 6 .] Studies in Immunity : Theory of an Epidemic. 513 
practically. It concerns the method by which smallpox infection 
is spread from smallpox hospitals. Many believe that the infection 
passes aerially from the infected centre, so that in a district in 
the immediate vicinity of this, case after case occurs due to this 
cause. Now, it cannot for an instant be denied that there is often 
in the neighbourhood of smallpox hospitals an amount of smallpox 
quite out of proportion to that in the rest of the locality, but that 
this is necessarily due to aerial infection is a position far from 
tenable. 
In the first place, it has been seen that a small amount of 
infection placed anywhere in a suitable locality is capable of 
producing a large epidemic ; and though all the cases of the fever 
be promptly removed from the district, the disease may continue 
to spread and an epidemic develop with the typical distributions 
in time and space ( vide diagrams for London, 1902). It is in 
relation to this type that the causes of a smallpox epidemic must 
be investigated. 
Secondly, some centres of a town are, for reasons more or less 
unknown, much more suitable for the spread of an epidemic than 
others ; and though the disease may first start in the part more or 
less distant from this, yet, as the epidemic proceeds and infection 
is introduced into this district, the cases there may so increase 
as to ultimately make this the chief centre of the outbreak. So 
that it must always be borne in mind that a local outbreak in the 
neighbourhood of an hospital may be as much an accident of place 
as a result of the proximity of the hospital. 
Lastly, the form of the epidemic wave in time is of importance. 
If smallpox be introduced into a new district at a late period in the 
epidemic wave, when the organism has lost its infectivity to a 
certain extent, then we would, a priori , expect that an epidemic 
of shorter duration would result, unless the new locality prove 
fitted to temporarily rejuvenate the organism. 
The epidemic in Liverpool in 1901-2 is recorded with the 
greatest care and detail in a report by Dr R. Reece, which was 
presented to the Local Government Board and published this year. 
This report is very suitable for the present purpose. Full details 
as to the dates of the occurrence of the cases, and fortnightly maps 
showing distribution of these cases in the city, are given. All the 
PROC. ROY. SOC. EDIN. — YOL. XXYI. 33 
