The circumstances of typhus and relapsing fevers need no general stating after what has been said of particular epidemics in England and Scotland, or remains to be said, for the most distinctive instances of all, in the chapter on fevers in Ireland. There has been so little typhus in the country at large since the disease began to be registered apart in the mortality returns, in 1869, that hardly anything can be inferred except the fact of its disappearance. It is significant, however, that Sunderland, one of the two great towns which have kept typhus longest and in largest measure (Liverpool being the other) is distinguished for the overcrowding of its dwelling-houses (7·24 persons to a house in the Census of 1881, 7·00 in the Census of 1891). But the circumstances of enteric fever are not only not so obvious as those of typhus in the historical way; they are also more complex and disputable. One fact in the natural history of enteric fever has been made clear in the chronology, namely, its greater frequency after a severe drought. It was in the autumn of 1826, after the driest and hottest summer of the century, that cases of fever with ulceration of the bowel were first described and figured in London. It was in the autumn of 1846, after the next very dry and hot summer, that cases of the same fever again became unusually common in many parts of England and Scotland. The same sequence has been remarked on more recent occasions and in various countries. It is explained by taking into account some other facts in the natural history of enteric fever. In nearly all countries in our latitudes, autumn is its principal season, and autumn is the season when the level of the water in the soil, or in the wells, is lowest. Virchow states the law of enteric fever in the following simple and concrete way: “We [in Berlin] have a certain number of cases of typhoid at all times. The number increases when the sub-soil water falls, and decreases when it rises. Every year, at the time of the lowest level of the sub-soil water, we have a small epidemic.” A sharp rise above the mean level of the year, from the first week of September to the end of October, has been well shown for London from the admissions to the hospitals of the Metropolitan Asylums Board, 1875-1884. The curve has an equally sharp descent, passing below the mean line of the year in the second week of December[403]. There are indications that it is the partial filling of the pores of the sub-soil with water, after they have long been occupied with air only, that makes the virus of typhoid active, or, in other words, that the rains of late summer and autumn are the occasion of the seasonal increase of the infection.