26 



TUBERCULOSIS, HEREDITY 



probability. If we start with a given population and a 

 given number of centres of infection, we find the number 

 of cases either tends to increase or to decrease according 

 to the ratio of cases to population, but after a certain 

 period approaches a finite asymptotic value — what we 

 may term the stable value. 



The history of an infectious disease is represented by 

 a diagram of this kind : 



Fig. IV. Units of Time. 



It does not tend to increase or decrease indefinitely. 

 Which case arises depends on the proportion of centres 

 of infection to total population at the start. The number 

 of cases then remains stable until some marked change 

 in environment or ratio of population to infection-centres 

 starts the process afresh. 



Now something precisely of this kind, this ' natural 

 history' of an infectious disease, is to be expected in 

 the case of tuberculosis, if there be a real hereditary 

 immunity. Do we find it ? 



To answer this let us look at the diagrams for male and 

 female deaths. The first diagram represents the general 

 death-rate corrected for age distribution in the population. 



