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252 A ship disease.— Ship epidemics were common occurrences formerly. The ideal haunt of yellow fever is the low-lying, hot, squalid, insanitary district in the neighbourhood of the wharfs and docks of large seaport towns. A place disease.— It is, in a sense, a place disease like malaria. That is to say, if the patient be removed to a hitherto unaffected spot, his attendants and neighbours will not contract the disease, unless the spot itself first become infected. The occurrence of this place infection will depend on whether the particular locality affords in sufficient numbers the appropriate intermediary— the stegomyia mosquito— for the transmission of the germ brought by the patient. If the locality supplies this condition, then, for the time being, the disease will spread and become epidemic; if the locality does not supply this condition, then the disease will not spread. It is safe, therefore, to visit a yellow fever patient if he is lodged outside the endemic or epidemic area; but it is never safe for the susceptible to visit the endemic area, whether they come into direct contact with the sick or not.

Immunity acquired by prolonged residence or by a previous attack.— A well-established fact about yellow fever is that the natives of, and those who have lived for a long time in, the endemic area are practically immune from the disease; or, if they are attacked, the disease is usually of a very mild type. It is also said that if the native quits the endemic area his immunity decreases in proportion to the length of time he remains away; so that, after long absence, on his return to the endemic area he may be attacked just as an ordinary visitor might be. Further, those who enter the endemic area for the first time are the most susceptible, the susceptibility decreasing with length of residence. It has been inferred from this, perhaps —but not certainly— correctly, that at such places as Rio and Havana the endemicity of the disease was kept up by the continual influx of foreign and, therefore, susceptible visitors. Race as influencing susceptibility.— Formerly it was believed that the negro is little liable to yellow fever,