But _morbus pauperum_ is not the only principle of infective disease. There are pestilent infections which do not come readily under the law of poor, uncleanly and negligent living, in any ordinary sense of the words; and there are some communicable diseases which directly contradict the principle that infection falls upon those who engender it by their mode of life. Unwholesome conditions of living may be trusted to engender disease, but it does not follow that the infection so engendered will fall upon those who lead the unwholesome lives; sometimes it falls upon the class who are farthest removed from them in social circumstances or domestic habits, or who are widely separated from them in racial characters. This principle I believe to be not only a necessary complement to the more obvious rule, but to be itself one of wide application. It has been an original theme of my own in former writings, to which I take leave to refer in a note[529]; and, I have now to try here whether it may not suit the rather paradoxical and certainly mysterious circumstances of the sweating sickness on its first outbreak in the autumn of 1485. If the insanitary state of London were insufficient to explain the engendering of the disease, the next thing is to look for a foreign source. Suspicion falls at once upon the foreign mercenaries who landed with Henry Tudor at Milford Haven on the 6th of August. Who were these mercenaries? Did they suffer from any contagious disease? Were they likely to have engendered the sweat? Can the infection be traced, in matter of fact, to them? In seeking an answer, it will be necessary to enter somewhat fully into the history of the expedition.