It was Sydenham's intimate friend Robert Boyle who worked out the hypothesis of subterraneous miasmata as a cause of epidemic (and endemic) diseases. An account of his theory will be found in the chapter on Influenzas and Epidemic Agues. It may be said here that it needs only a few changes, especially the substitution of organic for inorganic matters in the soil, to bring it into line with the modern doctrine of miasmatic infective disease as expounded by the Munich school.
It has not been usual to think of spotted fever, (or of influenzas), in that connexion; but a telluric source of the epidemic constitution of 1685-86 was clearly Sydenham's view; and as the fever came in circumstances like those of the last great plague, and was thought at the time to be the forerunner of another great plague, its connexion with recondite decompositions in the soil, dependent on the phenomenal drought of two whole years before, cannot be set aside as a possibility, the less so that the fever, although of the type of typhus, was not a fever of cold, hunger, and domestic distress, but mainly of the warm, or mild, or soft weather following the long drought, and of many well-to-do-people, as in the great Netherlands fever of 1669. My view of it is that it was the modified successor of plague, the _pestis mitior_, which used to precede and accompany the plague, now become the dominant constitution. The authentic figures of its mortality come from London; but Sydenham says that its "effects were felt far more in other places"; although Short's abstracts of parish registers, given above, do not indicate excessive mortality throughout England.
Retrospect of the great Fever of 1623-25.
The most instructive instance of _pestis mitior_ in Britain is not the pestilential fever which led up to the last plague (1665-6), but the great epidemic of fever all over England and Scotland which reigned for two or three years before the great outburst of plague in 1625. I go back to this because it was not wholly or even mainly a famine fever (although it was as general as one of the medieval famine-fevers), and because in that respect it furnishes a close parallel to the fever of 1685-86, which I regard as the successor of the plague. After this interlude in the history, we shall proceed to consider the question of the final extinction of plague.