England or Scotland its origin has usually been traced to the "Emerald Isle," where frequent epidemics have occurred during the present century; that of 1826 to 1828 attained considerable proportions, and that of 1846 to 1847 was the most severe of the present century. The number of cases in the last-mentioned epidemic in Ireland is estimated to have been over a million, or about one in seven of the population.
Typhus has prevailed at various times in Mexico, Peru, and Chili, as a result of importation from Spain, since the year 1570, when it first appeared in Mexico. Its first introduction into the United States was at a much later date, and corresponds with the period of extensive emigration from Ireland to the United States and Canada during the present century. For the most part the disease has been confined to the emigrants themselves, or to their immediate attendants on board ship, at quarantine stations, or in the isolation hospitals to which the sick have been transferred. Although the disease has very frequently been brought to our seaport cities, it has rarely extended to the resident population of these towns, and is unknown in the interior of the country. In New York the disease spread to some extent in 1818, 1827, 1837, and 1847, and in Philadelphia a considerable epidemic occurred in 1836, and again in 1862 to 1864.
The conditions governing the epidemic prevalence of relapsing fever are very similar to those mentioned in connection with the ætiology of typhus. It is especially liable to prevail during times of scarcity of food, and indeed epidemics are very frequently coincident as to time and place with those of typhus. It is, no doubt, transmitted by personal contagion, and its prevalence is therefore largely influenced by circumstances relating to the susceptibility of individuals, their sanitary surroundings, and their aggregation in ill-ventilated apartments. It attacks more especially those individuals in infected districts who occupy the densely populated and filthy portions of towns and cities, and, as stated by Engel, is peculiarly a morbus pauperum, or disease of the poor. On the other hand, it is quite independent of climatic influences, and, so far as we know, has no definite local habitat. The specific germ of this disease was discovered by the German physician Obermeier in 1873. It is a slender spiral filament, endowed with very active movements, and is found in the blood of relapsing-fever patients during the primary febrile paroxysm and also during the subsequent relapses which are characteristic of the disease.
It is impossible to say when or where relapsing fever had its origin, but our first reliable accounts of the disease date from the early part of the eighteenth century, when it prevailed as an epi-