XX11 INTRODUCTION. 



has been communicated from some other person suffering in a similar manner. 

 When small-pox, for example, breaks out in a house we all believe that it has been 

 caught from some one else, and if we fail to discover the mode of communication we 

 see no grounds for altering our opinion, but believe that in that individual case our 

 information is defective. The majority of contagious affections with which we have 

 to deal are communicated from one human being to another, whilst a few, such as 

 hydrophobia and glanders, are communicated by the lower animals. It has been 

 suggested that possibly the poisons of some contagious diseases may be derived from 

 plants, but no conclusive evidence has been adduced in support of this view. The 

 contagious particles must exist under many different forms, and be given off in 

 many different ways. Some affections are caused by obvious parasities, and of this 

 we have familiar examples in itch and ringworm. Most of our contagious poisons 

 have no palpable existence, but are given off in the various exhalations and 

 excretions of the bodv, but especially in those emanating from the lungs and skin. 

 Some are supposed to exist in the breath alone, as in the case of whooping-cough, 

 whilst others seem to be present in all the exhalations. Hydrophobia is an instance 

 of a contagious malady transmissible only through a special secretion the saliva. 



Many contagious affections are conveyed from one individual to another without 

 the necessity for any immediate contact between them. The contagion is given off 

 into the surrounding atmosphere, and thus passes to the unaffected person, being 

 inhaled, or swallowed, or absorbed by the skin. Diseases that can be thus com- 

 municated are said to be " infectious," whilst the term contagion is usually limited 

 to instances in which the disease is communicated by actual contact. The contagious 

 principle often becomes attached to articles of clothing, bedding, hair, and so on, and 

 in this way disease is propagated. These particles retain their vitality or activity 

 for immense periods of time, and may in this way originate several epidemics at 

 long intervals. Persons passing between the sick and healthy often carry a conta- 

 gions disease to the latter. A contagious poison may also be conveyed by clothes 

 sent to the wash, or sent home from an infected school, or by letters, cabs, and 

 numerous other agencies. The careless manner in which many people help to pro- 

 pa^ate infections diseases is something startling. We recently heard of the case of 

 a lady who left the bed-side of a child suffering from scarlet-fever, took a cab, and 

 went to church, probably sitting side by side with healthy unprotected persons. The 

 contagions principle often becomes attached to furniture, or to the floor and walls of 

 rooms, and thus infection may arise after an indefinite interval, if the precaution 

 has not been taken of having the apartments properly disinfected. It has been 

 asserted that flies and other insects may be the means of disseminating contagious 

 disease--, by alighting first on infected and then on healthy individuals, and such a 

 mode of propagation is quite possible. There are marked differences with regard 

 to the facility and certainty of transmission of contagious diseases. For example, 

 scarlet fever is less contagious than measles or whooping-cough, but far more so than 

 either typhus fever or diptluheria. The probability of a contagious disease being 

 communicated is indirect proportion to the dose, that is to the quantity and strength 

 of the poison which reaches the system, but it must l>e remembered that in many 

 instances a very small quantii y -Mlliees. Most contagious poisons are destroyed by 



