It was long ago pointed out by certain French observers that newcomers are much more liable to be attacked by typhoid fever than persons who have lived for some time in an infected locality. In 129 cases examined with reference to this point by Louis, the patients in 73 had not resided in Paris more than ten months, and in 102 not more than twenty months. Bartlett noticed that during an epidemic in Lowell which he had the opportunity of observing the disease attacked the recent residents in much larger proportion than the old. Liebermeister also calls attention to this peculiarity of the disease. Murchison's experience in reference to this point has been somewhat similar, for he found upon examination of the records of the London Fever Hospital that 21.84 per cent. of the patients admitted there for typhoid fever had been residents of London for less than two years. Almost all of these patients came, he says, from the provinces of England, and were in good health and comfortable circumstances at the date of their arrival in London and for some time after. Moreover, a large proportion of them were first attacked within a few weeks after changing their residence from one part of London to another. He also refers to instances in which successive visitors at the same house at intervals of months, or even years, have been seized shortly after their arrival with typhoid fever or with diarrhoea, from which the ordinary occupants were exempt. These facts indicate with sufficient clearness that habitual exposure to the causes of the disease confers, to a certain extent at least, an immunity from their effects, just as it does in the various forms of disease arising from malaria. It is not unlikely, as has been suggested by Wilson,[22] that one of the causes of the frequency of typhoid fever in the early autumn in our American cities among well-to-do people is to be formed in the circumstance that during an absence of two months or more in the mountains or by the sea they have to some extent lost the immunity acquired by habitual exposure to sewer emanations, and return to the atmosphere of the city unprotected. [Footnote 22: The occurrence of typhoid fever in the early fall among persons who have spent the summer out of town is, however, susceptible of another explanation. In many instances they have returned to houses which have been not only unoccupied, but closed, during several months, and which, in consequence of the more or less complete evaporation of the water in the traps of the drain-pipes, have been thoroughly permeated by sewer gas.]