"That epidemic diseases prevail most in certain countries, in certain districts, in certain towns, and in certain parts of the same town. They prevail most in those countries which are the least cultivated; in those districts which are the most woody, the most exposed to particular winds and to inundations; in those towns which are placed in low and damp situations, and which are unprotected from certain winds; in those streets and houses, and even in those apartments of the same house, which are the most low and damp, the worst built, and the least sheltered. "That epidemics commence, spread, and cease in a manner perfectly peculiar. They arise, for example, in some particular quarter of a town, and do not attack the other districts which happen to be nearest it in regular succession, but break out suddenly in the most distant and most opposite directions. People are attacked, _not in proportion as the inhabitants of the affected mix with the inhabitants of the unaffected places, but in proportion as the inhabitants of the unaffected expose themselves to the air of the affected places_. "That the termination of epidemics is peculiar, since they cease suddenly at the exact period when the greatest number of persons is affected by them, and when the greatest mortality prevails. This fact is inexplicable under the supposition that epidemics owe their spread from person to person. To suppose that a disease which is propagated by contagion can rapidly decline and even suddenly cease, just when most persons are affected and the mortality is greatest--that is, when the contagious matter is proved to be in its most active and malignant state--is utterly absurd. "That epidemics attack the same person more than once, and that relapses are frequent amongst those suffering from them, whereas contagious diseases seldom affect the same individual a second time, and relapses are most uncommon." From all this it will be clear that the object of these articles was to prove that all epidemics have their origin in the bad sanitary conditions (as we now say) of the places in which they arise. It happened then, as very frequently happens in all sciences when the time is ripe for a discovery, that those working in different fields of observation noticed, at the same period, the same facts--some, as for example Dr Maclean, in their posts of observation during the epidemics in distant countries; Dr Southwood Smith in the fever-haunts of London. But it remained for him, collecting together all the experience and generalising from it, to announce the law on which they depend.