534 THE TREATMENT OF DISEASES. 



form, is by no means of modern origin, for in the days of ancient Rome the 

 physicians considered that many diseases were caused by the presence of minute 

 animals in the body. In the Middle Ages it was imagined that these animals 

 could be seen flying through the air in dense clouds, and it was seriously proposed 

 to blow trumpets and fire guns, and make a great noise during the prevalence 

 of an epidemic, so as to frighten them away. It is almost needless to say that 

 the organisms which nowadays are supposed to be instrumental in the production 

 of fever are very minute, and that they can be seen only with high powers of the 

 microscope. 



A very marked peculiarity of the infectious diseases is what is technically known 

 as their specificness that is to say, that the same poison always gives rise to the 

 same disease. We can best illustrate this by an example. A man may be exposed 

 to cold and may be none the worse for it, or the result may be that he gets a cold 

 in his head, or a cold in his chest, or he suffers from colic or diarrhoea, or tooth- 

 ache, or rheumatism, or in fact any one of a great number of complaints. But 

 should the same man be exposed to the poison of scarlet fever, he either catches that 

 disease or nothing. It never, by any chance, results in small-pox or typhus fever, 

 or any of the other acute diseases, and this is what we mean when we say a fever 

 is specific. 



In all fevers there is a certain incubative period, or period of incubation during 

 which the poison remains latent in the system without producing any effects. You 

 sit up to-night with a person suffering from a fever, and you want to know how long 

 it will be before you can make sure that you have not caught the complaint. Un- 

 fortunately, this is a question which is not always very easily answered, for the 

 period of incubation is in many fevers very variable. In some cases all we can say 

 is that it maybe only a few days, or as many weeks. Small-pox m is the fever 

 respecting which we can speak with the greatest certainty, its period of latency being 

 fourteen days, or more accurately thirteen times twenty-four hours, from the moment 

 of taking the disease. In some fevers typhoid, for example it is difficult to fix 

 the exact date of the infection, and often quite as difficult to fix that of the com 

 mencement of the disease. 



Most of the idiopathic fevers are characterised by a rash or skin eruption. The 

 appearance of this rash usually enables us readily to distinguish one fever from 

 another. The rash does not appear in all fevers on the same day of the disease 

 that is, at the same time from the commencement of the illness. In chicken-pox the 

 rash comes out on the first day, in scarlet fever on the second, in small-pox on the 

 third day, in measles on the fourth day, in typhus fever on the fifth day, and in 

 typhoid fever about the end of the first week. This is the general rule, to which, 

 however, there are a good many exceptions. Thus, the rash of measles may coma 

 out on the third day or the fifth day, or even the first day, of illness. Some rashes are 

 much more punctual in their time of appearance than others ; for instance, typhus 

 nearly always comes out on the fifth day. Then, again, the rash in all fevers does not 

 first appear in the same situation. In chicken-pox it may appear on any part of the 

 body; in scarlet fever it sometimes comes out all over at once, but usually at first on 

 the side of the neck and upper part of the chest ; in small-pox it is first observed on 



