550 
A Study of the Effects of Diphtheria Isolation 
la precisely the same way a positive correlation between the isolation rate and 
the attack- or death-rates by no means justifies us in asserting that isolation is 
worse than non-etfective. It is conceivable that in the period or the district 
under consideration with an increasing isolation-rate there might be decreased 
immunity in the population, greater virulence of the disease, or even a limit 
to the available isolation accommodation, so that in the case of attacks of an 
epidemic nature the isolation 7^ate would not increase proportionately to the cases, 
or indeed might eveia diminish*. Further, if apart fi'om the changes in a single 
district, we consider a great variety of districts, it may chance that the greatest 
isolation-rate occurs in those districts where the disease has been found most 
prevalent, because it appeared the most obvious remedy, and thus a greater attack- 
or death-rate would be no real measure of the futility of high isolation. 
If, however, it should turn out that on the whole the higher isolation rate is 
associated with the higher attack-rate or the higher death-rate then it will be clear 
(i) that there is ground for demanding a closer investigation as to the advantages 
of isolation, and (ii) that we may be overlooking the real method, or at least one 
or more important factors, of the transmission of the disease. It is conceivable 
that isolation of all cases during attack may be of far less importance than isola- 
tion of certain special cases for a shorter or longer period well subsequent to the 
attack, and after they would normally have resumed their ordinary avocationsf. 
The main problems which arise are accordingly these : 
(i) Have isolation-, attack- and death-rates changed continuously with the 
time, and are the apparent correlations really suggestive of causal relationships ? 
(ii) Are associations between isolation-rate and attack- and death-rates really 
spurious arising from tlie fact that where the attack- and death-rates have been 
severe there the remedy which appeared nearest to hand was more isolation ? 
* For example, if there were only 100 hospital beds available, and out of 100 cases 50 were sent to 
hospital, the isolation-rate would be 50 but if in the next year there were 300 cases and all the beds 
were used, the isolation-rate would be only 33 %. Thus limited accommodation may tend to produce 
a negative correlation between isolation-rate and attack-rate, so that a positive correlation between these 
two rates may be of more importance than its apparent significance. It is extremely probable that 
some of the falls in isolation-rates are really due to an increase of incidence, so that the same 
percentage of cases cannot be met by the available hospital accommodation. 
t It is, on the hypothesis of natural selection, a plausible view that the parasites — including under 
this term all disease organisms — which ultimately survive must tend to become innocuous to their 
hosts, and thus the decreasing virulence of certain diseases may be accounted for. The organism is 
destroyed owing to the death of the host or its own death at his recovery, or it has been modified by 
selection so as to become innocuous to its host relative to his immunity. But immunity is a matter of 
personal equation, and thus the function of the " carrier " in preserving and spreading a conceivably less 
nocuous form of the organism becomes clearer. We are not unaware of the view that the organism 
remains the same, but that the immunity is increased owing to " practise " of the leucocytes, but such a 
view requires the assumption of inheritance of acquired characters to explain reduced disease virulence, 
and further compels us to assume two types of immunity, the one which destroys the organism, and 
the other which without modifying it, establishes so to speak a mutual modus vivendi. 
