There is indeed one point of view in which it may be proper, and even necessary, to consider the question of contagion with relation to that of quarantine. Assuming the existence of contagion, if it can be proved that quarantine, instead of affording any protection against contagion, absolutely fosters it, then the stronger the proof of contagion the more decisive the argument presented by it against quarantine; and it will be shown hereafter that this is the true and the only relation in which contagion stands to this question. There is no more reason why the controversy on contagion should complicate the question of quarantine than why it should continue to encumber the general subject of the removable causes of disease, from which efforts have long been made to disentangle it. The discussion whether epidemic diseases arise and spread from contagion or from common or specific poisons generated in the localities in which these pestilences first break out, has nothing whatever to do with quarantine, the sole inquiry with reference to this question being whether, however epidemic diseases arise, quarantine can prevent their introduction into a country or arrest their progress when there. Few will question that the progress of the opinion of observers in Europe during the last half-century has been steadily towards a material modification, if not an entire abandonment, of the doctrine of contagion with reference to the majority of epidemic diseases, taking the word contagion in its strict sense, that is, the communicability of disease exclusively by contact: direct, that is, with the body or breath of an infected person; or indirect, with something which an infected person has touched. Cholera may be taken as an example of the diseases of the epidemic class. When cholera first invaded Europe in 1831,[26] the belief in its contagious nature was almost universal, and in this country in particular there was scarcely a medical man who did not entertain this conviction; but as in India, where this disease is known, the belief in its contagious nature is universally abandoned, so in Europe it gradually diminished in proportion as opportunities of observing the disease increased; and now in Russia, Poland, Prussia, France, Belgium, and England, the contrary view, with few exceptions, is maintained. Footnote 26: See note p. 61.