There is nothing incredible in the supposition that these men had brought a disease into London although they had not themselves presented the symptoms of that disease. Such importations are not unknown; the mystery hanging over them does not make them the less real. A well-known instance is the St Kilda boat-cold, “the wonderful story,” as Boswell says, “that upon the approach of a stranger all the inhabitants catch cold,” a story which Mr Macaulay, the author of the _History of St Kilda_, had been advised to leave out of his book. “Sir,” said Dr Johnson, “to leave things out of a book merely because people tell you they will not be believed, is meanness: Macaulay acted with more magnanimity.” The St Kilda influenza has been amply corroborated since then by parallel instances from the more remote islands of the Pacific, and by striking instances in veterinary pathology. Among the latter may be quoted the instance which has been heard of in Shropshire, of “sheep which have been imported from vessels, although themselves in a healthy condition, if placed in the same fold with others, frequently producing sickness in the flock[534].” But there is an instance on a vast scale from the United States, the instance of Texas cattle-fever, which has recurred so often, and has been so closely watched on account of the disastrous loss which it causes, that there is no room left to doubt the reality of that mysterious form of contagion. I shall have to speak very shortly of the malignant fevers of the assizes, which spread from prisoners who were not known to be ill of fever; these incidents are historical from the year 1522, when an epidemic of the kind arose among the court and grand jury at the gaol delivery in the Castle of Cambridge. Lastly the history of yellow fever, as expounded in part in this volume, is an instance of a long-enduring infection arising from the circumstances of the African slave-trade, the negroes themselves having been racially exempt from the fever although they had been the source of the virus.