I90f — 6.] Studies in Immunity : Theory of an Epidemic. 501 
by the insusceptibility, the epidemic is only lengthened, not 
changed in form, and the course still represents an ascent of con- 
stantly increasing slope and a sudden drop to the original level. 
An example of this is given in the accompanying diagram (diagram 
XVII.), and it will be seen that the form bears no resemblance to 
any of the curves shown in the preceding pages.* Another factor 
is evidently necessary — and this is perhaps to be found in the loss 
of infecting power on the part of the organism. The rate at which 
this occurs must be according to some law, and it seems reasonable, 
Diagram XVII. 
-100- 
RD 
Diagran 
Epidem 
Infectivity 
Organism 
n of 
ic. 
j\ 
\ 
of 
Constant. 
[ 
\ 
O 
cn 
I / 
/ 
/ 
1 
DU 
/ 
■ 
\ 
j 
on 
/ 
\ 
\ 
/ 
\ 
as a hypothesis, to assume that the decline proceeds according to 
the terms of a geometrical progression. Thus, if at the end of 
* This form is sometimes seen when a case of measles develops in a ward in 
which a number of susceptible children are confined. Although the sufferer 
be moved at once, yet the infection is present so early in the disease that usually 
several others succumb. In one actual case, when there were fourteen sus- 
ceptible children in a ward, the epidemic developed in the following way. 
First one case ; at the end of incubation period three cases ; then a fortnight 
later seven ; leaving only three to develop the disease, and these all succumbed 
in the next fortnight, so that the epidemic came to an end from the cause dis- 
cussed and took the form of diagram XV. This, however, is under artificial 
conditions, and bears no resemblance to a natural outburst. 
