ACUTE AND CHRONIC DISEASES 257 



situations than in others. We, in England, contract almost all 

 our diseases in dwelling-houses or in places of popular assembly, 

 such as schools and churches. In the slums of cities where over- 

 crowding is greatest, the separation between the sick and the 

 healthy most imperfect, the dilution of the dosage with fresh 

 incoming air most incomplete, and the general nutritive and 

 sanitary conditions worse, selection is more stringent than in better 

 houses and in rural districts. Such very common diseases as 

 measles, chicken-pox, and whooping-cough are usually always 

 present (endemic) in towns, whereas in the country and even more 

 in remote islands, such as the Hebrides, they tend to be more 

 distinctly epidemic. That is, in towns the exposure of susceptible 

 people to large doses is very frequent, while in the country it is 

 more exceptional. 



428. We saw also that all microbic diseases may be placed in 

 one or other of two categories. On the one hand are those 

 diseases against which immunity cannot be acquired, and, on the 

 other, those against which it may be acquired acquired immunity 

 being an entirely specific reaction which follows individual experi- 

 ence of disease, and which in every case is of avail only against the 

 particular disease which has been experienced. It is quite distinct 

 from ' innate ' immunity, which (though it is also specific) does not 

 follow but precedes experience of the microbes, and which arises 

 therefore like eyes, ears, hair, and teeth, as a nutritional character. 

 Tuberculosis and leprosy are typical examples of diseases against 

 which immunity cannot be acquired. No doubt, in a sense, 

 immunity may be acquired against some of them ; that is, owing to 

 improved health and vitality, an infected person may recover and 

 live immune; but this is not what is meant by the technical expres- 

 sion * acquired immunity.' On the other hand, a person may be 

 ' innately ' immune against such typically acute diseases as measles 

 and influenza. That is, he resists infection though exposed to 

 doses which would give rise to disease in the ordinary individual 

 who has not acquired immunity. But in the case of such diseases, in 

 the vast majority of instances, people differ, not so much because they 

 resist infection, for nearly all contract each disease if sufficiently 

 exposed to infection, as because they recover more or less easily. 1 



1 Inborn immunity, though rare in measles and so rare in smallpox that 

 formerly when the disease was prevalent an unmarked skin was in itself regarded 

 as an exceptional beauty, is common enough in some acute diseases. Thus, 

 though constantly exposed to infection, I have never contracted diphtheria nor 

 scarlatina. Probably, in the case of such diseases, the two forms of immunity 

 are evolved concurrently. 



